If she had called me, I __________ her.

The correct answer is C: would have helped. This sentence is an example of the third conditional, which is used to talk about hypothetical situations in the past that did not happen, and their imagined results. It expresses regret or a missed opportunity.

The standard structure for the third conditional is: If + past perfect, would have + past participle.

• The first clause, "If she had called me," is correctly in the past perfect tense (had + past participle).
• The second clause, "I __________ her," requires the structure "would have + past participle." The past participle of "help" is "helped."

Thus, "I would have helped her" perfectly fits this grammatical rule, indicating that because she did not call, I did not help her.

Let's examine the other options:

• A: help is the base form of the verb and is used in the first conditional or simple present.
• B: would help is used in the second conditional (If + past simple, would + base verb), which refers to hypothetical situations in the present or future.
• D: helped is the past simple tense, which is not part of the third conditional's main clause structure.

  Understanding the specific tenses and structures for each conditional type is crucial for accurate usage.