Answer:
B
Step-by-step explanation:
In this sentence the adverb clause is: Although Denmark is located on a peninsula.
The subject of the sentences is country.
The verb is includes.
The adverb is also.
More than 400 islands is a prepositional phrase.
Adverb clauses can modify verbs, but they can also modify adverbs and adjectives, so it is always a good idea to identify the parts of speech of the sentence before deciding which one is modified by the adverb clause.