Which is the best example of commensalism? a) honey bees feed on the pollen of cornflowers and the flowers reproduce. b) barnacles ride on a gray whale’s back to obtain food, and the whale is not harmed. c) squirrels and chipmunks eat the same nuts in a forest where food is scarce. d) prairie dogs often collect and eat food in groups so they are better protected from enemies.

Commensalism is a situation in nature where there is an association between two organisms that benefits one but does not help nor harm the other.

Let's go through each example in order to find one with commensalism.

A). Honey bees that feed on the pollen of cornflowers and help the flowers reproduce. 

This is not a form of commensalism as both organisms mutually benefit from the association.

B). Barnacles ride on a gray whale's back to obtain food, and the whale is not harmed.

This is a form of commensalism. The barnacles benefit from riding on the whale's back as they obtain food. The whale is also unharmed in the process.

C). Squirrels and chipmunks eat the same nuts in a forest where food is scarce.

This is not a form of commensalism as neither organism benefits. There is an increased competition for food that harms both of the organisms.

D). Prairie dogs often collect and eat food in groups so they are better protected from enemies.

This is not a form of commensalism as the interaction of the organisms is within the same species. In order for commensalism to take place, two different species must interact. 

The best example of commensalism is B.