Them is a personal pronoun which is used in form of third person plural in an objective case. In this sentence, "we" is the subject and the blank is the object. From the given sentence, it is understandable that it is in plural form. Option A is correct.
Themselves, Himself and Herself are reflexive pronouns. Reflexive pronouns are words ending in -self or -selves that are used when the subject and the object of a sentence are the same (e.g., I believe in myself). Options B, C, and D are wrong.