Which of the following is an example of parasitism?

Parasitism is a relationship between two organisms where one organism benefits at the expense of the other. In this case, a tick feeding on a dog exemplifies parasitism because the tick derives nourishment and benefits from feeding on the dog's blood, while the dog suffers as a result of this interaction, potentially facing health issues such as blood loss, disease transmission, or skin irritation.

The other scenarios illustrate different types of ecological relationships. For example, the relationship of a bird building a nest in a tree represents commensalism, where the bird benefits from the tree as a nesting site, but the tree is neither helped nor harmed. The interaction of a bee pollinating a flower is an example of mutualism, as both species benefit—bees get food from the nectar, while the flowers benefit from the pollination. Lastly, the scenario of a lion and zebra sharing a water hole depicts competition rather than parasitism, where both animals are vying for the same resource without a direct benefit or detriment to one another based on their relationship. Therefore, the tick and dog scenario is a clear and direct example of parasitism.