Explanation:

The given sentence has an error in the usage of "if" and "supposing". Both these words cannot be used together in the same sentence. Hence, the correct answer is "Supposing if" which needs to be replaced with either "Supposing that" or "If".

The correct sentence can be written as:

- If it rains, what shall we do?
- Supposing that it rains, what shall we do?

Breaking down the sentence:

- Supposing if: This is incorrect usage since both "supposing" and "if" convey the same meaning and cannot be used together.
- It rains: This is the main clause of the sentence which indicates the condition.
- What shall we do?: This is the dependent clause of the sentence which asks the question about the action to be taken.

Correct sentence structure:

- Supposing that / If + main clause, dependent clause.

The correct usage of "if" and "supposing" is as follows:

- If: Used to indicate a condition that may or may not be true.
- Supposing: Used to introduce a hypothetical situation or to suggest an assumption.

Conclusion:

The correct sentence structure is important to convey the intended meaning. In this case, the correct usage of "if" and "supposing" is crucial for the sentence to be grammatically correct.