The question focuses on the correct formation of conditional clauses. Option C accurately uses the first conditional, with 'if + present simple' in the condition and 'will + base form' in the result. Options A, B, and D are incorrect. Option A incorrectly uses future tense in 'unless it will rain' when it should be present tense. Option B mixes a past perfect in the condition with a conditional in the result, should be 'If I had known, I would have told you'. Option D should use will instead of had. Therefore, the only correctly formed conditional clause is found in Option C. **Core Concept:** Conditional Clauses. **Solution Analysis:** We need to identify the correct structure of the first conditional clause, 'If + present simple, will + base form'. **Option Analysis:** - **A. Unless it will rain, we will go for a walk.:** Incorrect use of future tense after 'unless', should be present simple 'Unless it rains'. - **B. If I would have known, I would have told you.:** Incorrect use of would have, it should be past perfect - 'if I had known' - **C. If it is raining, I will stay inside.:** Correct use of the first conditional with present simple and will. - **D. Provided that you go, I will had come.:** Incorrect verb form, should be will come - and you have to use a present simple after provided. **Common Mistake:** Errors in combining the correct tenses in conditional sentences and using them incorrectly in the 'if' part of the conditional clause.