The biggest constraint of plant breeding is

The correct option is A availability of desirable gene in the crop and its wild relatives
Plant breeding is the process where the desired gene from the wild type is introduced in the plants. The genes can only be transferred from the wild type when it is a related species.

Here some of the factors that have to be kept in mind are-

the selection of the variety of crops and their genetic constituents.

The wild variety which is taken should be disease free.

So, the correct option is 'availability of desirable gene in the crop and its wild relatives'