Which of the following helps to improve the variety of the crop?

Hybridization is the process of crossing two different species of the same genus. The crops produced by hybridization are of improved variety, also called genetically modified crops can be produced by the insertion of genes with desired traits. 

Photoperiodism is the phenomenon in which the length of daylight influences plant growth and development.
Irrigation is the process of applying water to land to aid in crop growth.
Fertilizers are substances added to soil to provide plants with essential nutrients such as nitrogen, phosphorus, and potassium.

Photoperiodism, irrigation and fertilizers do not improve crop variety.