Read the desperate sticky note left on the fridge and choose the right word to start the sentence.

___ we are completely out of coffee, I am borrowing your emergency stash of energy drinks.

The correct answer is Since.

Since is a subordinating conjunction used to introduce an adverb clause of reason, especially when the reason is already known to the listener. When an adverb clause starts a sentence, we place a comma before the main clause. "Because of" requires a noun phrase, while "Therefore" and "So" are used to introduce results, not reasons.