The survival of a rare New Zealand species of mistletoe that produces spectacular sprays of scarlet flowers
is threatened both because their leaves are extremely tasty to a voracious opossum species and also because their flowers are pollinated by two species of birds whose populations are in decline.
A) is threatened both because their leaves are extremely tasty to a voracious opossum species and also because their
B) is threatened both because its leaves are extremely tasty to a voracious opossum species and because its
C) is threatened both because its leaves are extremely tasty to a voracious opossum species and also its
D) are threatened both because its leaves are extremely tasty to a voracious opossum species and its
E) are threatened both because their leaves are extremely tasty to a voracious opossum species and because their
If we compare first few words of answer choices,
In A, B and C, we have "is" and in D and E, we have "are". We have singular-plural verb error. The subject is "The survival', which is singular...
Eliminate D and E
In A, pronoun "their" refers to "New Zealand species of mistletoe", which is singular. ....Eliminate A
In C, "both" is followed by "because", "and" must be followed by "because" to maintain parallelism. Here "and" is followed by "its". We have parallelism error.
Eliminate C.
Hence B is the correct answer.