I don't work at home, ___ a rule.

• A. Like
• B. In
• C. On
• D. As

Answer: (D)

The idea here is the fixed phrase as a rule, which means “usually” or “generally.” In this sentence, using as a rule signals a general tendency about the speaker’s practices. The word as introduces the standard or principle being stated, so I don’t work at home, as a rule = I typically don’t work at home.

The other options don’t form a natural idiom for this meaning. Like a rule would suggest similarity to a rule rather than a general habit, and in a rule / on a rule aren’t used to express general tendency in English. You could say As a rule, I don’t work at home, and that keeps the same sense.