Which of the following is a key feature of a mayfly?

A key feature of a mayfly is the presence of two long, slender tails known as cerci. These cerci are an important identifying characteristic of mayflies and set them apart from many other insects. The structure of cerci can also play a role in their locomotion and sensory functions, aiding in their navigation and ecological interactions.

While the other characteristics mentioned may be found among various insects, they do not specifically define mayflies. For instance, mayflies typically have more elongated bodies and longer antennae, which further differentiate them from other insects with shorter antennae or rounded body shapes. Thus, the presence of the two cerci is significant in the classification and identification of mayflies within the broader group of insects.