Option 1 -> Incorrect tense combination; "was following" (past continuous) and "had not regretted" (past perfect) don't form a proper conditional structure.
Option 2 -> Correct third conditional structure (If + past perfect, would + have + past participle) expressing a hypothetical past situation and its unrealized consequence.
Option 3 -> Incorrect conditional form; mixing present perfect with simple future doesn't suit this context of past regret.
Option 4 -> Grammatically incorrect; "will have follow" and "will not have regretting" are not valid verb forms.
Hence, Option 2: had followed, would not have regretted -> This sentence expresses a third conditional, indicating regret about not following advice in the past. The speaker didn't follow the advice (past reality), and now regrets it because of the consequences they're facing today. -> correct