Answer:
D. bees pollinating flowers
Step-by-step explanation:
Mutualism refers to the ecological association between the organisms of different species wherein both the organisms are benefited by the association. Among the given examples, bees obtain nectar from flowers. When bees visit the flowers for nectar, the pollens are stuck to their bodies. These bees transfer the pollen to other flowers while visiting them. In this way, bees obtain their food from flowers while the flowers are being pollinated. Since both bees and flowers are benefited by the association, it is an example of mutualism.