They both get something useful from each other.
Option: A
Step-by-step explanation:
The rhinoceros enjoys relief from the insects, while the birds enjoy a meal. Thus, both help each other mutually.
Ox pecker birds also called tick birds,ride on huge African animals like rhinos and zebras that feed on external parasites like the bot-fly larvae and ticks. The ox peckers feast on the parasites they find, and they also lend the favor of raising a loud warning when a potential predator approaches. As the rhinos get relief from the insects, they allow these birds to sit on their backs.