The most useful classification of drug for medicinal chemists is `"……"`

Options

A. on the basis of chemical structure
B. on the basis of drug action
C. on the basis of molecular targets
D. on the basis of pharmacological effect

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the most useful classification of drugs for medicinal chemists. We need to evaluate the given options based on their relevance to medicinal chemistry.

2. **Evaluating the Options**:
- **Option 1: On the basis of chemical structure**: This classification groups drugs based on their chemical structures. While it is useful, it may not always indicate the drug's action or target.
- **Option 2: On the basis of drug action**: This classification focuses on how drugs act on specific biochemical processes. It is useful but does not provide insight into the molecular interactions.
- **Option 3: On the basis of molecular targets**: This classification is based on the interaction of drugs with specific biomolecules like proteins, nucleic acids, and lipids. This is crucial for medicinal chemists as it helps in understanding how drugs exert their effects at a molecular level.
- **Option 4: On the basis of pharmacological effect**: This categorizes drugs based on their therapeutic effects, which is more useful for clinicians than for medicinal chemists.

3. **Identifying the Most Useful Classification**: After evaluating all options, it becomes clear that the classification based on molecular targets (Option 3) is the most useful for medicinal chemists. This is because it allows them to understand the specific interactions between drugs and their biological targets, which is essential for drug design and development.

4. **Conclusion**: Therefore, the answer to the question is that the most useful classification of drugs for medicinal chemists is **"on the basis of molecular targets"**.