The correct answer is option 3: species.

In biology, species is a taxonomic rank that defines a group of organisms capable of interbreeding and producing fertile offspring. Horses and donkeys belong to different species, Equus ferus and Equus africanus asinus respectively. Even though they are closely related and can produce offspring, the resulting mule is infertile. This is because mules inherit different numbers of chromosomes from their horse and donkey parents, which makes it difficult for their reproductive systems to function properly.

Option 1: Class is an incorrect answer because horses and donkeys belong to the same class, Mammalia, along with other mammals.

Option 2: Older is an incorrect answer because the infertility of mules is not related to the age of the horse or donkey.

Option 4: Genus is an incorrect answer because horses and donkeys belong to different genera, Equus and Equus asinus respectively, but their infertility is still due to belonging to different species.

In summary, the infertility of mules is because horses and donkeys belong to different species, and their reproductive systems are not compatible enough to produce fertile offspring.