In docking, a molecule that binds to a receptor is called a

In molecular biology and biochemistry, docking refers to the process where two molecules fit together in a specific way, often involving a smaller molecule binding to a larger one.

• A. Ligand: A ligand is a molecule that binds to another molecule, typically a larger one, to form a complex. This is a general term used for molecules that bind to receptors.
• B. Enzyme: An enzyme is a protein that catalyzes biochemical reactions. While enzymes bind to substrates, the term "enzyme" describes its function, not just its binding property to a receptor in general.
• C. Gene: A gene is a segment of DNA that carries genetic information. It is not a molecule that binds to a receptor in the context of molecular docking.
• D. Template: A template is a molecule that serves as a pattern for the synthesis of another molecule (e.g., DNA template for replication). This is not the correct term for a molecule binding to a receptor.

Based on the definitions, a molecule that binds to a receptor is called a ligand.

The correct answer is A.
$\boxed{\text{A. Ligand}}$