Haploid sperm and egg are produced via meiosis. what process is responsible for producing the 2n zygote?

The answer is fertilization.

Fertilization is the process in which two haploid gametes fuse to create one diploid cell. In this process, male sperm fuses with female egg which creates a zygote. Since they are haploid (n), after fertilization, they will create the zygote which contains chromosomes from the sperm and chromosomes from the egg. The zygote, thus, has two sets of chromosomes, so it is diploid (2n).