An allosteric enzyme usually has
A. One active site
B. One active site and one allosteric site
C. Active site and two types of allosteric sites
D. Two types of active sites

Hint:Allosteric enzymes are unique to other enzymes because of their ability to adapt different environmental conditions due to their special characteristics.

Complete answer:
An allosteric enzyme is an enzyme which, after bonding an effector, changes its conformational set and results in an obvious change in the binding affinity of another binding site. This "action at a distance" by the binding of another ligand at a distinctly different position is the nature of the allosteric principle. In many important biological processes, allostery plays a vital part, including but is not limited to cell signaling and metabolism control. Allosteric enzymes do not need to be oligomers as before, and several systems have actually shown an allosteric inside a single enzyme. In biochemistry the regulation of a protein by binding an effector molecule to a site other than the active site of an enzyme is Allosteric regulation.

The substrate is used to integrate the enzyme into a complex of enzyme-substrate. The active site is defined as the part of the enzyme in which substrate molecules are bound. As a result of its interaction with the enzyme active site, the substrate is converted into the product. A molecule that is not a substrate can bind to enhance or reduce the activity of a substrate in the allosteric site of the enzyme. An allosteric regulator binding changes the enzyme's form and affects its overall function. Allosteric enzymes are unique to other enzymes because of their ability to adapt different environmental conditions due to their special characteristics. It contains an allosteric site above the active location which binds the substrate to the special characteristic of the allosteric enzymes.

Thus, the correct answer is option B. i.e., One active site and one allosteric site.

Note:Allosteric enzymes do not need to be oligomers as before, and several systems have actually shown an allostery inside a single enzyme. In biochemistry the regulation of a protein by binding an effector molecule to a site other than the active site of an enzyme is Allosteric regulation.