The survival of a rare New Zealand species of mistletoe that produces

Approach

1. "The survival" is Singular, So there should be a singular verb "is" instead on "are". Negate D & E
2. Notice that we have idiom of type "Both...and...", Correct usage is "both X..and Y". Negate C - It skips "because" after ..species and...
3. Option A has 2 issues -> 1. Usage of "their" for species, whereas "its" is the correct usage ...why? in case you get confused while solving, hint is already in sentence where "The survival of a rare New Zealand species of mistletoe that produces. 2. Usage of "also" after and which disturbs the idiom "both X..and Y" usage. Although, option A can be negated straightaway with above step. Just to have full clarity on this part i added my thoughts here. Negate A
4. Option B is the correct answer

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

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.

"both because X and because Y" should be the correct idiom. The subject noun here is singular "survival"

A) is threatened both because their leaves are extremely tasty to a voracious opossum species and also because their -- "their leaves" wrongly refers to singular mistletoe

B) is threatened both because its leaves are extremely tasty to a voracious opossum species and because its -- CORRECT, parallelism mentioned above is maintained and singular "its" refers to mistletoe

C) is threatened both because its leaves are extremely tasty to a voracious opossum species and also its -- "both because and also" is wrong idiomatically

D) are threatened both because its leaves are extremely tasty to a voracious opossum species and its -- "survival" is singular, hence "are threatened" is wrong.

E) are threatened both because their leaves are extremely tasty to a voracious opossum species and because their -- same as D

Answer is B

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.

'And' in option B already serves 'because' to the second part of the sentence too. Parallelism is there even without the second because.
Though it is the best option available, it isnt upto the mark.

A) is threatened both because their leaves are extremely tasty to a voracious opossum species and also because their
"and also" is redundant
"their" is plural and Subject is singular

B) is threatened both because its leaves are extremely tasty to a voracious opossum species and because its
Correct

C) is threatened both because its leaves are extremely tasty to a voracious opossum species and also its
"and also" is redundant
Parallelism not maintained "because its x and because its y"

D) are threatened both because its leaves are extremely tasty to a voracious opossum species and its
Subject is singular
Parallelism not maintained "because its x and because its y"

E) are threatened both because their leaves are extremely tasty to a voracious opossum species and because their
Subject is singular

survival is the subject and is singular. Hence the verb should be 'is', not 'are'.
idiom = BOTH X and Y. Here, X and Y should be parallel and logically similar.

Both X and Y is the correct idiom
a) c) d) are out unidiomatic form usage
Subject - The survival(singular)
e) uses plural verb for singular subject