slabhyankar,
Should you refer to The authority on matters of English grammar, namely The Oxford University Press,
you will learn the vital difference between types of phrases and the function they perform.
The phrase itself is is a prepositional one.
Its function may be adjectival or adverbial, depending on the role in plays in a sentence.
Remembering that an adjective modifies a noun or pronoun.
and an adverb modifies a verb, adjective or another adverb,
There is the slight matter of the non-existant sentence with which to come to a conclusion..
In any case, 'in the morning' is actually an idiomatic phrase,
and the adverb, when required is 'mornings'.
"If that is what is to be learnt as English grammar, I certainly do not want to pass any examination in such kind of English grammar"' iti slabhyankar.
If you have no formal qualifications in English, let alone in the teaching profession, your postings and bloggings serve only to pour darkness upon the Sanskrit faculties.
Taff_Rivers