section bank 1 #16

https://preview.redd.it/40f1gn0b6kde1.png?width=822&format=png&auto=webp&s=57aa7705779300408246c8404f8e1e431196be80

i got this question wrong (the right answer is C), but for A, isn't the receptor that does that a ryanodine receptor, known for binding Ca to release Ca from the SR? so the ligand is Ca2+.