The first choice on a dichotomous key is 1a. Are wings covered by an exoskeleton vs 1b. Are wings not covered by an exoskeleton? Which organism is separated from the other by this question?

D. Ladybug

Explanation:

A dichotomous key consists of a series of choices which help in identification of an organism. Every choice leads to another question till the end of key.

Here, the first question in the key is about the presence or absence of exoskeleton on wings of the organisms. Four organisms are given. Out of them only ladybug has wings covered by exoskeleton so it is separated from rest of the organisms like housefly or dragon fly. Lady bug will be selected in 1a key. Rest of the organisms will be selected by 1b key. 1b will lead to more questions which will further differentiate between the remaining organisms.