What type of reproduction requires the union of male and female gametes?

Sexual reproduction is characterized by the fusion of male and female gametes, known as fertilization. In this process, the male gamete (sperm) and the female gamete (egg) combine to form a zygote, which then develops into a new organism. This method introduces genetic diversity, as the offspring have a unique combination of genes from both parents, which can enhance adaptability and survival in changing environments.

In the context of the options provided, asexual reproduction does not involve gametes and typically results in offspring that are genetically identical to the parent organism. External fertilization is a method that can occur during sexual reproduction but specifically refers to the situation where fertilization occurs outside the body, commonly seen in many aquatic organisms. Budding is a form of asexual reproduction where a new organism develops from a part of the parent organism, independent of gametes.

Therefore, sexual reproduction is the accurate term for the process that requires the union of male and female gametes.