Biology

The correct answer is: B.Ligand.

Explanation:

A ligand is a molecule that binds to a receptor (often a protein) on the surface of a cell membrane. Once bound, the ligand typically triggers a signaling event without entering the cell itself. This binding initiates a response within the cell, such as activating intracellular signaling pathways.

Here’s why the other options are incorrect:

A.ATP: ATP (adenosine triphosphate) is the primary energy carrier in cells, but it is not specifically a molecule that binds to surface proteins as a signaling mo...