She has two sons,---are college students.

肯定选A.这是个定语从句,whom代表the two sons,而且both是个复数概念.
B 也是个定语从句,但是neither得用单数,后面的are不合适.
C和D都不属于定语从句了,如果是个独立的句子当然没问题,但问题是,这两句之间是逗号,这就意味着属于一个句子,只能用定语从句这类的复合句才行.
如果把逗号改成句号,就得选C了.或者：加个and连接两句话,就得这样说：
She has two sons,and both of them are college students.