Correct Answer: (b)
Solution:As per Subject-Verb concord, a singular subject takes the singular form of verb and a plural subject is conjugated with the plural form of verb.
In the sentence, 'layers' is plural in 'layers of sediments", thus 'build (without the ending 's') will be used with it.
The correct sentence is 'Over long periods of time, layers of sediments build up to a height of few kilometres."