Identify which of the following is a complete and balanced chemical equation

because a complete equation must have all the elements present on the left hand side of the arrow on the right hand side of the arrow as well, and to be balanced there must be the same number of each type of atom, so both A and C are full equations, but A is not balanced because although there are two oxygen atoms on each side of the arrow, there is only one hydrogen atom on the left and two on the right, thus the right hand side is 'heavier' than the left and the equation is unbalanced.

Explanation:

answer: the metamorphic rocks are formed when they are exposed to high pressure and high temperatures. this happens when one of the earth's plates comes in contact with the other plate by traveling in opposite directions, straight into each other or slipping under one another. the friction that is caused by this activity results in   melting of rocks and when it cools they are known as metamorphic rocks. this process needs a controlled amount of pressure and temperature otherwise it may turn into magma.