During which type of reproduction do male and female sex cells fuse?

Sexual reproduction involves the fusion of male and female sex cells, known as gametes. In this process, the male sperm and female egg come together to form a zygote, which then develops into a new organism. This type of reproduction introduces genetic diversity, as the offspring inherits a combination of genes from both parents. This genetic variation is important for the evolution and adaptability of species.

In contrast, asexual reproduction does not involve the fusion of gametes and typically results in offspring that are genetically identical to the parent. External fertilization refers to a specific method within sexual reproduction where fertilization occurs outside the female's body, usually in an aquatic environment, but it still falls under the umbrella of sexual reproduction. Cloning, on the other hand, is a method of producing genetically identical organisms without the need for gamete fusion. Thus, the correct answer that specifically refers to the fusion of male and female sex cells is sexual reproduction.