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.
<em>'Each of the students'</em> has the same meaning as '<em>each student</em>', 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, <em>'someone'</em> is third person singular, and therefore must be followed by a singular verb (<em>arrives</em>, not <em>arrive</em>).
