Answer:
The answer is: B. Each of the students plans to graduate this spring.
The subject-verb agreement means that the subject and the verb have to agree in number - both have to be either plural or singular.
The only sentence where this happens is B. Each of the students plans to graduate this spring.
'Each of the students' has the same meaning as 'each student', hence the subject is singular. The verb is third person singular (he/she/it plans), which means that they agree in number.
This is not the case for the rest of the sentences. For example, in sentence A, 'someone' is third person singular, and therefore must be followed by a singular verb (arrives, not arrive).
