The correct answer is B. adjective phrase.

While writing my report, I found the information on this website to be very helpful.

In order to find out what kind of phrase ‘on this website’ is, we have to ask what its function in the sentence is. In this case, it is describing something, saying that it is on this website. Now, we have to determine what it is describing, because adverbs and appositives also describe. In order to find out what it is describing, ask yourself ‘What is ____?’.

What is on this website?
Information is on this website.

Therefore, ‘on this website’ is describing ‘information’. Because ‘information’ is a noun, the phrase ‘on this website’ must be an adjective phrase, because adjective phrases describe nouns.

An example of an adverb phrase would be:

The dancers moved very gracefully.
The dancers did what very gracefully?
The dancers moved very gracefully.
Therefore, ‘very gracefully’ is an adverb phrase because ‘moved’ is a verb. This is also the case when the phrase is describing an adjective or another adverb.

Appositive phrases are difficult because they also describe nouns; however, appositives are separated from the rest of the sentence by a set of commas, and serve to rename the noun they are describing.

Here is an example:

The girl, my cousin, walked over to me.
Who is my cousin?
The girl is my cousin.
Therefore, ‘my cousin’ is describing ‘girl’. Because girl is a noun, it is either an appositive phrase or an adjective phrase. Since ‘my cousin’ is separated from the rest of the sentence by commas and is renaming the noun, ‘girl’, it must be an appositive. Therefore, ‘my cousin’ is an appositive phrase.

Hope this helps!