The Sun keeps shining a) mostly by gravitation contraction b) mostly by chemical reactions c) by converting hydrogen to helium. d) by converting hydrogen to carbon. e) by E = mc2. f) by F=G M_1 M_2/R12

C) by converting hydrogen to helium.

Explanation:

The Sun keeps shining because of nuclear fusion processes, which consists in creating helium atoms by combining hydrogen atoms, of an atomic mass less that of helium, in conditions of high core pressure and temperature. In other words, the Sun keeps shining by producing helium at the expense of its hydrogen reserves.

Therefore, the correct answer is C.