Supposing if / it rains / what shall / we do?

Answer: Option A

The error is in Option A: Supposing if.
The word "supposing" and "if" have the same meaning in this context.
Using them together is redundant (unnecessary repetition).
You should use either "supposing" or "if," but not both.
For example, you can say: "Supposing it rains, what shall we do?" or "If it rains, what shall we do?"
So, the correct option is A: Supposing if because it contains the error.