Scientists classify rocks a. by composition and texture. c. by mass. b. by volume. d. by color and size.

  Composition 

The chemicals of the minerals in the rock determine the type of rock. Also formation, density, texture, and other factors have a role, but the composition is the driving classifier.

 i think it would be a. I know this is a late answer but i need points. 
 

hope this helps anyways.