A ligand is observed to bind its receptor on the outside of the cell. As a result of this binding, a chemical process is initiated within the cytoplasm on the inside of the cell. The original ligand was most likely a

Substrate inhibitor, activator or a neurotransmitter.

Explanation:

The original ligand was most likely a substrate inhibitor, activator or a neurotransmitter. A ligand is a small molecule that is able to bind to proteins by weak attractive forces such as ionic bond and hydrogen bonds etc. It also transmit signals in the process from one part to another part. It also act as inhibitor and activator of reactions in the process.

the answer is "a.) well–developed vascular system."

explanation: