The number of free electrons in the outer shell of a diamond’s carbon atom is . . . . . . . . A. zero B. two C. four D. three

The correct answer is: C. four

A carbon atom has four valence electrons, which are located in the outer shell. These electrons are free to move around and can be shared with other atoms, forming covalent bonds. This is what makes diamond such a strong material.

Option A is incorrect because a carbon atom has four valence electrons, not zero.

Option B is incorrect because a carbon atom has four valence electrons, not two.

Option D is incorrect because a carbon atom has four valence electrons, not three.