The correct option is
B nephew
Answer : B
Explanation:
The sentence thus talks about the relation between the speaker and the speaker's sister's son.
Thus for the speaker, it is his "nephew".
Nephew means a son of one's brother or sister, or of the brother-in-law or sister-in-law.
The other options are incorrect because:
Uncle means a brother of one's mother or father. Thus in this question, for the nephew, the speaker is his uncle.
Niece means daughter of one's brother or sister or one's brother-in-law or sister-in-law.
Bride is the word for woman on her wedding day.
Therefore the correct complete sentence is;
He's my sister's son. So, he's my nephew.