The process in which two gametes fuse to make one cell is called
asexual reproduction
fertilization
conjugation
pollination

The answer is fertilization.

Fertilization is the process in which two gametes fuse to creates one cell. In this process male sperm fuses with female egg which creates a zygote. 

Asexual reproduction involves the formation of offsprings without gamete fusion. Conjugation is a process of exchange genetic material between bacteria. Pollination is a process of transfer pollen to the female reproductive structures in plants, which enables fertilization.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.