Concept:This question tests the correct use of past conditional structures and tense matching.
When a past unreal condition is stated, the factual result must also be expressed in the past tense.
Explanation:The opening sentence, "If it had rained, we would have had to postpone the concert", is a third conditional.
It describes an imaginary past situation where rain caused a postponement.
The next clause begins with "It didn't rain", which states a past fact.
Therefore, the result clause must also state a past reality: no postponement was actually required.
Option A, "we won't have to", uses the future tense, so it does not match the past context.
Option B, "we would have had to", repeats the unreal conditional, but it contradicts the fact that the rain never came.
Option C, "we wouldn't have to", refers to present or future obligation, not a past outcome.
Option D, "we didn't have to", is the simple past and correctly expresses that no postponement was necessary.
Only Option D agrees with both the tense and the meaning of the statement.
Answer:D. we didn't have to