"The water in this well is fairly deep."

The correct answer is Option (3) → Adverb of Degree

An adverb is a word that tells how, when, where or how much something happens. It can also describe an adjective or another adverb, for example she sings sweetly, very hot.

In this sentence, fairly comes before the adjective “deep”. Fairly means quite, but not very. It tells how deep the water is.

Option	What it tells	Example
(1) Adverb of Reason	why something happens	It rained heavily and therefore the match was put off.
(2) Adverb of Time	when something happens	My cousin will reach Delhi tonight.
(3) Adverb of Degree (answer)	how much or to what extent (very, quite, fairly, too): it often comes before an adjective	The test was fairly easy, so most of us finished early.
(4) Adverb of Manner	how an action is done: here “fairly” describes the adjective “deep”, not an action	Grandpa walks slowly in the evening.