"He failed the exam because he didn’t study."

The correct answer is Option (4) → Adverb of Reason

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.

The underlined words because he didn’t study are a clause (a group of words with its own verb) doing the work of one adverb. They give the reason he failed the exam.

Option	What it tells	Example
(1) Adverb of Manner	how something is done	She folded the clothes neatly.
(2) Adverb of Place	where something happens: no place is named in the underlined words	The cat is sleeping upstairs.
(3) Adverb of Time	when something happens: the underlined words give a reason, not a time	I met Rohan yesterday.
(4) Adverb of Reason (answer)	answers the question why?: such a clause often begins with because, as or since	We stayed indoors because it was very hot.