The correct option is C Would help
Sentences that begin with 'If I were...' present a Type 2 conditional. That is a present hypothetical condition. You use it when you want to imagine that the present moment, now, is different than it really is.
'If' is a conjunction that joins two clauses together. In Type 2 conditional sentence, the structure in terms of tense is:
If + simple past, would (verb) (If + were, would + help)
Hence the underlined verb phrase "would have helped" isn't correct here, it needs to be changed.
The tense of "would have helped" is in the past perfect tense, whereas the required form must be in the present tense (help). Hence D is incorrect
Option C: 'Would help' is the correct form as the structure requires, hence C is correct.
Option A: 'Am capable of helping' does not present a subjunctive mood i.e. a hypothetical situation. But, the sentence does, hence A is incorrect.
Option B: 'Could have helped' means that something was possible in the past, or something had happened in the past, but you didn't have the ability to perform a certain action.
The given situation isn't the same, hence B is incorrect.