Which of these is the best definition for disintegration?
a. when rock becomes moist
b. when rock is physically broken into smaller pieces
c. when rock is chemically broken into smaller pieces
d. when rock undergoes a rather large chemical change

Both B and C are correct. Both physical and chemical processes can act separately or, more often together to achieve disintegration of rocks. Disintegration of rocks by mechanical (physical) means depends on the application of force. In nature the sources of such forces include thermal expansion and contraction of rock, freezing and thawing, and the growth of plants and organisms in rock. Due to such forces, the rock breaks into smaller pieces without changing its chemical composition. Disintegration of rocks by chemical means happens as a result of exposure to solution such as acidic rain, carbonation, hydration and oxidation by circulating water which leads to a change in composition of the rock and a break down in its form. Due to this, the texture of the rock changes and may become brittle and easily break into smaller pieces.