The coenzymes `NAD^+`, FAD and `NADP^+` are :
(a) allosteric enzymes
(b) metal ions
(c) cofactors
(d) both (a) and (c)

Options

A. allosteric enzymes
B. metal ions
C. cofactors
D. both (a) and (c)

Explanation

To solve the question regarding the coenzymes NAD+, FAD, and NADP+, we will analyze each option provided:

1. **Understanding Coenzymes**: Coenzymes are organic molecules that assist enzymes in catalyzing reactions. They are not proteins themselves but work closely with enzymes to enhance their activity.

2. **Evaluating the Options**:
- **Option (a) Allosteric enzymes**: Allosteric enzymes are a specific type of enzyme that can be regulated by molecules that bind to sites other than the active site. Coenzymes like NAD+, FAD, and NADP+ do not fit this definition, as they do not regulate enzyme activity in this manner.

- **Option (b) Metal ions**: Metal ions are inorganic cofactors that assist in enzyme function. However, NAD+, FAD, and NADP+ are organic molecules and not metal ions.

- **Option (c) Cofactors**: Cofactors are non-protein chemical compounds that are required for the biological activity of a protein. Since NAD+, FAD, and NADP+ are indeed organic molecules that assist enzymes, they are classified as cofactors.

- **Option (d) Both (a) and (c)**: Since we established that NAD+, FAD, and NADP+ are not allosteric enzymes, this option cannot be correct.

3. **Conclusion**: Based on the analysis, the correct answer is **(c) cofactors**.

### Final Answer:
(c) cofactors