A bullet is fired horizontally out of a gun at the same time another is dropped from the end of the barrel. The one to hit the ground first...

They both hit the ground at the same time.  So your answer is C. Gravity works on the bullets just the same regardless of velocity.  The bullets have to be the same calibre and weight for this to work.

Newton was the first person to study gravity seriously, and he came up with the law of universal gravitation:

Each particle of matter attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

The standard formula for gravity is:

Gravitational force = (G * m1 * m2) / (d2)