Fill in the blank in the following sentence with the most appropriate…

The correct answer is Option (2) → conscience

Conscience is a noun meaning the inner sense of right and wrong that guides a person. The blank follows "my", so it needs a noun, and acting by one’s own sense of right and wrong is what brings "immense peace of mind".
Example: His conscience would not let him keep the wallet he had found.

Why the Other Options are Incorrect?

Option 1: conscious — Incorrect. Conscious is an adjective meaning aware or awake; an adjective cannot follow "according to my". (Conscious and conscience sound alike: conscious is an adjective, "aware"; conscience is a noun, "the sense of right and wrong".)
Example: The patient was conscious throughout the operation.

Option 3: consensus — Incorrect. Consensus means general agreement within a group; it is not a personal guide to one’s actions.
Example: The committee reached a consensus after a long debate.

Option 4: conscientious — Incorrect. Conscientious is an adjective meaning careful and hard-working; it cannot stand alone after "my".
Example: She is a conscientious student who never misses a deadline.