If industrial pollution continues to deplete the ozone layer, the resulting increase in ultraviolet radiation will endanger human health, causing a rise in the incidence of skin cancer and eye disease, and perhaps even threatening global ecological systems.
(A) and perhaps even threatening
(B) and may even threaten
(C) and even a possible threat to
(D) as well as possibly threatening
(E) as well as a possible threat to
yup, B for parallelism.
A may mislead at first glance coz of "causing" . IMO, "causing a rise... eye disease" works as a participle phrase to clarify how the radiation "endanger human health" rather than forms a parallel action to " endanger" ---> its gerund form can't be counted in the parallelism.
"threaten ...." must be parallel to "endanger" --> threaten must be in infinitive form ---> A and D out for the use of "threatening" ... C and E out for the use of the noun form of "threat" .
B stands the best.