The public's widespread belief in the existence of
UFOs and their general curiosity about extraterrestrial life has generated considerable interest in science fiction
The subject for the helping verb "has" is "the public's widespread belief"
in the existence of UFO's and their general curiosity about extra terre. life--Prepo Phrase.
So this is the reason it can not be a compound subject, because the actual subject is "The public's widespread belief"
A. UFOs and their general curiosity about extraterrestrial life has
their has no antecedent
B. UFOs and they are generally curious about extraterrestrial life which has
they has no antecedent
C. UFOs as well as their general curiosity about extraterrestrial life, have
their-no antecednet have should be has
D. UFOs as well as its general curiosity about extraterrestrial life, has
its-public has verb is correct so we keep it for the time being
E. UFOs as well as general curiosity about extraterrestrial life,have
have is wrong here.
So option D finally