Deep space telescopes are fitted with the faint object cameras, that are way better than ground-based spectroscopically functioning telescopes. The Subaru telescope at Hawaii observed the faintest object ever from a ground-based telescope. But when compared to other deep-space telescopes like Hubble's deep space telescopes it's very small. They have reached up to a magnitude of 31 , which observes 20 times fainter objects .
the world's current population is 7.3 billion and the world population in 1915 was 1.8 billion. The increase over the past century is 7.3 billion - 1.8 billion = 5.5 billion. So, the correct answer would be "more than 5 billion" (option D).