The correct option is A zygote
Reproduction is the process by which organisms give birth to offspring which ensures the continuity of the species. It can be of two types- asexual (only one parent is involved) and sexual (usually two parents) are involved. In the case of sexual reproduction, gametes (sexual reproductive units) are formed. They can be divided into male gametes and female gametes. Sperm and pollen grains are male gametes, while egg is a female gamete. When compatible male and female gametes fuse, they give rise to zygote which is capable of becoming an independent individual after proper development. So, the vital link that ensures continuity of species between organisms of one generation and the next is zygote.