Double fertilization is a fusion of

The correct option is D One male gamete with egg and other with secondary nucleus
Double fertilization is a complex fertilization mechanism of flowering plants or angiosperms. This process involves the joining of a female gametophyte of megagametophyte with two male gametes of sperms. One sperm is fertilized the egg cell and the other sperm combines with the two polar nuclei of the large central cell, also called the secondary nucleus of the megagametophyte.
Thus, the correct answer is option A.