"which is located between Libya and Morocco" is a nonessential adjective clause.
This clause is an adjective clause because it describes or gives further characteristics to the noun "Algeria". And it is nonessential since it is between commas and if you take it out of the sentence, the whole sentence would still have full meaning.
The other options are incorrect because unlike nonessential clauses, the essential ones do not require commas and are necessary to understand the sentence, and the given phrase does contain a relative pronoun: which.
