The public's widespread belief in the existence of UFOs and their general curiosity about extraterrestrial life has generated considerable interest in science fiction.

A. UFOs and their general curiosity about extraterrestrial life has
Here, their is refering to public. It is wrong. Public is singular.
Also, when you have and you need need to use plural verb.
x and y are not is

B. UFOs and they are generally curios about extraterrestrial life which has
Similar error as A. Eliminate.

C. UFOs, as well as their general curiosity about extraterrestrial life, has
Their is plural. Eliminate.

D. UFOs, as well as its general curiosity about extraterrestrial life, has
When you use as well as, in addition to and other additive phrases, you need to look at the subject before additive phrases.
It is just fluff. You can ignore it. So public is singular so its verb will be has.
Correct answer

E. UFOs, as well as general curiosity about extraterrestrial life, have
As well as is an additive phrase.
Public is singular. have is wrong