Which question would be asked in a dichotomous key?
a. how long are the insect’s antennae?
b. are the insect’s antennae longer than its head?
c. are the insect’s antennae long or short?
d. how does the insect use its antennae?

The most appropriate answer would be B. Are the insect’s antennae longer than its head?

Dichotomous key refers to the tool or method which helps the scientist to identify organisms.

It uses a specific series of questions which help in segregation of organisms and ultimately help the scientist to identify the organism on the basis of the observations or properties share by the organism.

Example of a simple dichotomous key is shown below.

Similarly, insects can have antennae longer or shorter than their heads. Thus, the question "Are the insect’s antennae longer than its head?" would help in segregating insects based on their antennae length.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: