The answer is B. In a biological reaction involving an enzyme, the active site of an enzyme binds to the substrate.
The active site of an enzyme is a specific region of the enzyme where a substrate molecule binds. This binding creates an enzyme-substrate complex, which helps to facilitate a chemical reaction. The active site of an enzyme is typically a three-dimensional structure with a particular shape that is complementary to the shape of the substrate molecule. This ensures that only specific substrates can bind to the active site and helps to increase the specificity of the reaction.
During a reaction, the enzyme may modify the substrate molecule in some way to create a new product. Once the reaction is complete, the product is released from the enzyme, and the enzyme is free to bind to other substrate molecules.