The correct option is D Adverb Clause
A clause is a part of a larger sentence containing a subject and a predicate of its own.
A Principal Clause is an independent clause which can exist even without a subordinate clause. A Subordinate Clause is called a dependent clause because it cannot exist without the help of the principal clause. It begins with a subordinating conjunction.
In this sentence, 'You may sit' is the principal clause and 'wherever you like' is the subordinate clause.
The subordinate clause modifies the verb 'may sit' in the principal clause like an adverb does. It answers that the person may sit in his choice of place. Hence, its a subordinate adverb clause.
Thus, Option D becomes correct and the other three options cancel out.