Options B and C: As mentioned in the line "mother remarried a clergyman", Sir Isaac Newton's mother remarried a clergyman (in the past tense). This action has already taken place. Hence 'remarries a clergyman' is incorrect; 'remarries' is in the simple present tense. The simple present tense is used to indicate a daily habit. (It can also be used to indicate an event in history to make it sound more dramatic. But, the given paragraph is in past tense.)
'Remarried' is in the simple past tense. It indicates an action that happened in the past.
Hence C is the correct option and B is incorrect.
Option A: 'Was remarried a clergyman' is grammatically incorrect, because the preposition 'to' is missing. The right form should be: 'was remarried to a clergyman'. (Also, 'was remarried...' is in passive voice whereas the details mention the action 'to remarry' in the active form. The passive voice indicates that the subject 'mother' receives the action, she was made to marry, whereas the active voice indicates that the subject performed the action.)
Option D is incorrect because the answer lies only in C.