(b, d)

Receptors are proteins that are crucial to the body's communication process. The majority of these are embedded in cell membranes. They are embedded in the cell membrane in such a way that their small part possessing active site projects out of the surface of the membrane and opers on the outside region of the cell membrane

In the body, the message between two neurons and that between neurons to muscles is communicated through certain chemicals. These chemicals, known as chemical messengers are received at the binding site of receptors proteins.

To accommodate a messenger, the shape of the receptor site changes. This brings about the transfer of messages into the cell. Thus, the chemical messenger gives a message to the cell without entering the cell.