Desired improved varieties of economically useful crops are raised by ?

The correct option is C Hybridization
Hybridization is the process of interbreeding between individuals of different species (interspecific hybridization) or genetically divergent individuals from the same species (intraspecific hybridization). Crop yields increase dramatically when hybridization is used to exceed one or more of the parents in size and reproductive potential.

Thus, the correct answer is option C.