This structure carries the electrical signal of contraction to the calcium gates on the calcium..?

Your answer has to be D because it intes into the cytoplasm of the muscle cell, forming membranous tubules called transverse tubules (T-tubules). Terminal cisternae is the enlargement of smooth endoplasmic reticulum found in muscle cells on either side of the transverse tubules. The triad of transverse tubules surrounded by two smooth ER cisternae transmit altered membrane permeability down the tubules. And acetylcholine is a chemical (neurotransmitter) not a structure.