What slows down the contraction of a star-forming cloud when it makes a protostar? (a) production of fusion energy (b) magnetic fields (c) trapping of thermal energy inside the protostar?

The correct answer for this question is the option c: (c) Trapping of thermal energy inside the protostar.

When astronomers say "protostar" they refer to the early stage in the formation of a star. This celestial objects are very difficult to detect, because they are balls of gas and dust.