Answer:
D. any biting insects other than horseflies are strongly attracted to zebras.
Step-by-step explanation:
In order to assess the plausibility of the zoologist's hypothesis, it would be most helpful to know whether if any other insects other than the horseflies are strongly attracted to the Zebra. If other insects are attracted to the zebra, we can then try to research and see if these other insects also show tendencies to be attracted to horizontally polarized light before we can conclude on this hypothesis. The hypotheses will hold more strongly if the same phenomenon applies to other biting insects attracted to the Zebra.