A: May I bring a radio on a camping trip?
B1: No, you must bring it.
B2: No, you must bring one.
(Which answer is right? Do we have to use 'one' or 'it' as the answer to the question?)
Either answer is self-contradictory.
May I bring a radio on the camping trip?
Yes, you may bring one.
(Oh, I made a mistake. It is a little cold in my country. My hands feel a little cold, so it's a little difficult to type on the keyboard.)
A: May I bring a radio on a camping trip?
B1: No, you must not bring it.
B2: No, you must not bring one.
(Do you mean 'one' is right?)
Between these two choices, B2 is better. Either one is grammatical, but B2 is better.
The correct answer to the question "May I bring a radio on a camping trip?" would be B2: No, you must bring one.
The use of "one" in this response emphasizes the need to bring a radio, any radio, without specifying a particular radio or model. This means that you are required to bring a radio with you on the camping trip.