When we want to transfer a single gene from undesirable variety into a well adopted agronomically desirable variety the most suitable breeding method is

When we want to transfer a single gene from an undesirable variety into a well-adopted agronomically desirable variety the most suitable breeding method is Back cross.
It is the mating of a hybrid organism (offspring of genetically unlike parents) with one of its parents or with an organism genetically similar to the parent.
The backcross is useful in genetics studies for isolating (separating) certain characteristics in a related group of animals or plants.