Is the word questioning an adverb?

No, "questioning" is not an adverb. It is a present participle acting as an adjective.

Here's why:

* Adverbs modify verbs, adjectives, or other adverbs. They answer questions like "how?", "when?", "where?", or "to what extent?".

* Adjectives modify nouns or pronouns. They describe a noun or pronoun.

In the sentence "He had a questioning look on his face," "questioning" modifies the noun "look" by describing the kind of look it was.

Let's look at some examples:

* Adverb: "He walked quickly to the store." ("quickly" modifies the verb "walked")

* Adjective: "The blue car is mine." ("blue" modifies the noun "car")

* Present participle as adjective: "The questioning child asked many questions." ("questioning" modifies the noun "child")

So, "questioning" in this context acts as an adjective because it describes a noun.