During which type of reproduction do cells from two parents unite?

Sexual reproduction is characterized by the union of gametes from two distinct parents, leading to the mixing of genetic material. In this type of reproduction, typically, a male gamete (sperm) from one parent merges with a female gamete (egg) from the other parent. This fusion forms a zygote, which develops into a new organism that has genetic contributions from both parents.

This method of reproduction enables genetic variation in the offspring, which is advantageous for evolution and adaptation to changing environments. It contrasts with asexual reproduction, where a single organism or cell divides to produce offspring that are genetically identical to the parent, resulting in less genetic diversity.

The other options, such as binary fission and vegetative propagation, exemplify asexual reproduction methods. Binary fission is a process commonly seen in prokaryotic organisms like bacteria, where a single cell divides into two identical cells. Vegetative propagation involves a single parent plant producing new individuals through structures like runners or tubers, again leading to offspring that are genetically identical to the parent. Thus, sexual reproduction is the only option that involves the contribution of genetic material from two parents.