Concept:Subject-verb agreement: a plural subject must take a plural verb.
Explanation:The sentence has three underlined parts: (a), (b), and (c).
Part (b) contains an error in subject-verb agreement.
The subject is "layers of sediments" — the word "layers" is plural.
The verb "builds up" is singular, which does not match the plural subject.
The correct plural verb form is "build up".
Parts (a) and (c) are grammatically correct.
The corrected sentence reads: "Over long periods of time, layers of sediments build up to a height of a few kilometers."
Answer:(b)