The correct option is B c
The option that has an error, is the correct option.
It can be noticed that parts (a) and (b) do not have any error. They are grammatically correct and form an if-clause that is complete in meaning.
Hence options A and B are not the right answers.
The given sentence is a conditional of Type 3. This means that when the if-clause is in the past perfect tense ('you had just hinted' is a verb form in the past perfect, these words are coherently placed), the resultant clause will be a perfect conditional (would have + past/present participle).
In option C, the verb form 'would...help' is the error. As explained earlier, the result or main clause will be a perfect conditional. So the verb form will be: would + adverb (most certainly) + have + past participle (helped)
Hence, option C is the correct option with the error.
Option D is incorrect because the sentence has an error (c).