Question

The laws of thermodynamics state that, in a heat engine, _____.

Answer 1

Answer: The correct option is "all the heat energy from a source cannot be converted to mechanical energy"

Explanation:

Heat engine is a system which converts heat energy to the mechanical energy. During this process, a lot of heat energy is lost to surrounding.

Heat engine is a device that uses heat to do work. It is impossible to extract an amount of heat from the reservoir at high temperature and use this heat to do some work.

Some heat is rejected to the reservoir at low temperature.

Therefore, the laws of thermodynamics state that, in a heat engine, all the heat energy from a source cannot be converted to mechanical energy.

Answer 2

There are four laws of thermodynamic which define and characterize the thermodynamic system at thermal equilibrium. 
The laws of thermodynamics state that, in a heat engine,  all the heat energy from a source cannot be converted to mechanical energy.