The process in which pressure from upper layers of sediment pushes down on the lower layers , causing the sediments to stick toghter and form solid rock

a. compaction
b. cementation
c. limestone
d. coal

I believe the answer is A. Compaction;
because the actual definition is: The process by which the porosity of a given form of sediment is decreased as a result of its mineral grains being squeezed together by the weight of overlying sediment or by mechanical mean; which is fairly similar to what is already stated in the question.