The correct answer is NEITHER.
It is not a relative pronoun, because it is not a pronoun but a preposition. And it is not a subordinating conjunction as it is not subordinating a clause into another clause. It is not even introducing a clause.
The phrase "about the fire drill" is a necessary complement to the verb TELL. This verb (in this case in the Simple Past) selects two objects: one necessarily is the person to whom the action is directed (in this case "me") -better called Indirect Object pr Recipient of the action- and the other one is the Direct Object, that is to say the thing being told (in this case "about the fire drill").
