Allosteric enzymes possess

The correct option is "Active site and an allosteric site".

To understand why this is correct, let us break down the structure and function of an allosteric enzyme step-by-step:
1. Active Site: Like all enzymes, allosteric enzymes possess an active site. This is the specific region where substrate molecules bind and undergo a chemical reaction.
2. Allosteric Site: In addition to the active site, allosteric enzymes possess a unique, physically distinct regulatory site called the allosteric site. The word "allosteric" comes from the Greek words allos (meaning "other") and stereos (meaning "solid" or "space"), referring to this "other site" on the enzyme.
3. Regulation: Effector molecules (inhibitors or activators) bind specifically to this allosteric site. Binding induces a conformational (structural) change in the enzyme, which alters the shape and affinity of the active site, thereby regulating the enzyme's catalytic activity.

Thus, by definition, the fundamental characteristic of an allosteric enzyme is the presence of both an active site (for catalysis) and an allosteric site (for regulation).