Choose the most appropriate pair of phrases from the options to fill…

The correct answer is Option (2) → had followed, would not have regretted

Third conditional (an imagined past):

• Use: the speaker did not follow the advice and now regrets it. An imagined past that did not happen is expressed with the third conditional.
• Pattern:
  • If-clause: if + past perfect (had + past participle): "If I had followed your advice".
  • Main clause: would + have + past participle: "I would not have regretted it".
• With "today", the mixed form "would not regret it today" is also common, but it is not among the options.
• Complete sentence: If I had followed your advice, I would not have regretted today.

Why the Other Options are Incorrect?

Option 1: was following, had not regretted — Incorrect.

• The main clause of a conditional needs would. "Had not regretted" is a plain past perfect.
• "Was following" does not express an unreal condition.

Option 3: have followed, will not regret — Incorrect.

• It mixes the present perfect with the future and gives no clear conditional meaning.
• A real future condition would be "If I follow your advice, I will not regret it".

Option 4: will have follow, will not have regretting — Incorrect.

• Both forms are ungrammatical: "will have" needs a past participle (followed, regretted).
• "Will" is never used in the if-clause of such a sentence.

Contrast: an unreal past takes had + past participle and would have + past participle. A real future condition takes the present and will (If I follow your advice, I will not regret it).