Coenzyme is an organic substance that is bound to an enzyme and essential for its activity. Let's understand this in detail.

Enzymes and their activity:
Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They are proteins that have a three-dimensional structure, which enables them to bind to specific substrates and catalyze chemical reactions. However, enzymes alone are not always sufficient to catalyze a reaction. They require the help of non-protein molecules called cofactors, which can be either inorganic (such as metal ions) or organic (such as coenzymes).

Organic cofactors:
Organic cofactors are small, non-protein molecules that bind to enzymes and are required for their activity. They are often derived from vitamins, and some of them are used as dietary supplements. Organic cofactors can be further classified into two types: prosthetic groups and coenzymes.

- Prosthetic groups: These are tightly bound to enzymes and are essential for their activity. They are usually covalently attached to the enzyme's protein structure and cannot be easily removed. For example, heme is a prosthetic group found in the enzyme hemoglobin, which is responsible for carrying oxygen in the blood.
- Coenzymes: These are loosely bound to enzymes and can be easily removed or transferred between enzymes. Coenzymes often participate in the reaction along with the enzyme and help in the transfer of chemical groups or electrons. Examples of coenzymes include NAD+ and FAD, which are involved in redox reactions, and ATP, which is involved in energy transfer.

Holoenzyme and Apoenzyme:
An enzyme that has both its protein component (apoenzyme) and organic cofactor (coenzyme) is called a holoenzyme. The cofactor is an essential part of the holoenzyme and is required for its activity. On the other hand, the apoenzyme alone is inactive and cannot catalyze the reaction.

Therefore, the correct answer is option B. Coenzyme is an organic substance bound to an enzyme and essential for its activity.