The answer is C. Hope that helped!
Gorbachev's reforms are ultimately responsible for the Soviet collapse, which saw the end of Soviet superpower status, a massive reduction in the Soviet military's size and strength, the unilateral evacuation of all territories in Central and Eastern Europe occupied at great human cost in the Second World War, and a rapidly declining economy fragmented into fifteen separate states. Much of the argument that the Soviet political system and economy needed reform needed change to avoid collapse came directly from him - the phrase "Era of Stagnation" to describe the Brezhnev years is actually a piece of Gorbachev's rhetoric. However there seems to be a strong case (made by Stephen Kotkin in Armageddon Averted), that while the Soviet economy was growing at ever slower rates, and increasingly unable to close the ever-present gap in living standards between the USSR and the West, probably could have continued to muddle on - there was no imminent danger of political and economic collapse in 1985. It's also important to note that Gorbachev's reforms did not cause the collapse of the USSR on purpose, and Gorbachev was always committed to maintaining the union in some reformed shape under an economic system that was still socialist. However, his reforms both began to pick apart the centralized economy without really creating new institutions, which caused severe economic disruptions, and his political reforms unleashed new political movements outside his control, while all of these reforms antagonized more hardline members of the nomenklatura (party establishment). Ultimately he lost control of the situation. The Soviet system was highly-centralized and governed in a top-down approach, and it was Gorbachev who put reforms into motion and also removed members of the Soviet government and Communist party who opposed reforms. Gorbachev's period tends to get divided into roughly three periods: a period of reform, a period of transformation, and a period of collapse. The period of reform lasted roughly from 1985 to 1988, in which Gorbachev and his supporters in the government (notably Eduard Shevardnadze, Gorbachev's foreign minister and the future President of Georgi, and Aleksandr Yakovlev, Gorbachev's ally on the Politburo and the intellectual driver of reforms) tried a mixture of moderate reforms and moral suasion to revitalize the Soviet economy as it was, echoing Khrushchev's reforms of 20 years previous. While the goal was a revitalization of Soviet society and the economy, there was a very strong focus on morality: this period notably featured the anti-alcoholism/prohibition campaign, and very public campaigns against corruption (Dmitry Furman called this a "sort of Marxist Protestantism"). When these efforts did not secure the results that Gorbachev and his reformers desired, more far-reaching reforms were pursued in the 1988-1990 period. This is when Gorbachev made massive changes to Soviet foreign policy, such as withdrawing from Afghanistan in 1989, announcing unilateral cuts to military spending and forces at the UN in 1988, and more or less cutting the USSR's Eastern European satellite states in 1989. On the domestic sphere, this is when Gorbachev pushed through major political changes to the Soviet system, pushing through a new Congress of People's Deputies to be filled through semi-free elections, removing the Communist Party's monopoly of power and creating the office of President of the USSR for himself in 1990. This is also the period when glasnost ("openness", ie the lifting of censorship) took off, and these all were largely attempts to establish a new base of support for continued reforms once it became clear to Gorbachev that most of the Communist Party was uninterested in this. These reforms ushered in the 1990-1991 chaos, at which point Gorbachev essentially lost control. Falling oil prices and the crackdown on alcohol sales (which were a massive part of the Soviet budget), plus Gorbachev's loosening of management and sales restrictions on state firms while maintaining most of their subsidies, plus plans for importing of new Western machine tools and technology to revitalize the economy, seriously destabilized the Soviet budget, and caused the government to turn to the printing presses to cover ever increasing deficits.
The Vietnam war was in favour of the Northern revolutionary forces. Just after the war, a peace treaty was signed which allows a government to be initiated in the Southern Vietnam, to find out that it has become a puppet government of the North, therefore, Vietnam remained to be a communist state.
