The sentence that contains an error in subject-verb agreement is the following one: Most of the class eat lunch at school.
Explanation:
Subject verb agreement means that the verb and the subject must agree in number. This means that both need to be singular or both need to be plural. In the sentence above, the subject and the verb do not agree in number. In order to identify this problem, you need to identify the subject of the sentence.
The subject of the sentence is the NP "most of the class". The verb should agree in number with the noun head, which is "class". "Class" is a collective noun, so you should take into account that it can be used to refer a group as a unit or to its members as individuals. In this case, the noun refers to whole group, so the verb should be treated as singular and should contain the -s. The revised version of the sentence would be as follows: Most of the class eats lunch at school.
