Final answer:
The agreement error in sentence 3 of the specified paragraph is a number agreement issue, where 'half the students' is a plural noun and the verb should be 'eat' instead of 'eats'.
Step-by-step explanation:
The cause of the agreement error in sentence 3 of the paragraph is related to the word number. Referencing 'half the students', which is a plural noun phrase, the verb should also be plural to maintain proper subject-verb agreement. However, 'eats' is a singular verb, so it should be corrected to 'eat' to correctly align with the plural subject 'half the students'. Thus, the correct answer to the question is b. number.