I think the correct answer is receptor. To respond to a chemical signal a cell must have a receptor and also this where the signaling molecule can bind. A receptor is a protein molecule that can receive signals from the outside environment of the cell.
A is your answer. it cant be B because H doesn't equal 2 on both sides.it cant be C because S doesn't equal 3 on both sides of the equation . It also cant be D because Au doesn't equal 3 on the first part of the equation so your best answer is A. Hope this helps!!!!
We need the context of the work of you want us to answer
