Is whenever the shipment arrives a subordinating conjunction or a relative pronoun?

The Word 'whenever' is functioning as a conjunctionjoining the sentence 'the shipment arrives' to the previous part of a compound sentence.

Example: We will call you whenever the shipment arrives.

The Word 'whenever' is also an adverb and 'whenever the shipment arrives' can also be considered an adverbial clause, modifying the verb 'will call'. But that was not one of the choices.

The Word 'whenever' is not a relative pronoun.

The relative pronouns are: who, whom, whose, which, that.