The most useful classification of drugs for medicinal chemists is
On the basis of chemical structure
On the basis of drug action
On the basis of molecular target
On the basis of pharmacological effect
As we have learnt,
One of the methods of Classification of drugs is done on the basis of molecular targets.
Drugs possessing some common structural features may have the same mechanism of action on targets. The classification based on molecular targets is the most useful classification for medicinal chemists.
Hence, the correct answer is Option (3)