The correct answer is A. Mrs. Baroda was greatly tempted that night to tell her husband — who was also her friend — of this folly that had seized her.
Step-by-step explanation:
The em-dash (—) which is longer than the regular dash is often used to enclose comments, remarks or similar information that is additional information or does it is not essential in the sentence. Also, in this cases, an em dash should be included at the beginning or end of the remark, comment, etc. According to this, the correct sentence is the one in option A because in this sentence, the section "who was also her friend" acts as extra information or a remark, and therefore this should be enclosed using the em-dash.