Correct answer: B

A: Incorrect because of idiom and modifier. Idiom error: the correct idiom is ability + infinitive, not "ability of." Modifier error: "because of having the ability" is awkward and does not clearly attribute the ability to the cacti.
B: Correct. (B) clearly states that "they" (cacti) have an ability to retain moisture.
C: Incorrect. Modifier error: "In" does not explain the cause/effect relationship between the cacti's ability to retain moisture and their ability to survive with little rain.
D: Incorrect. Modifier error: "Having their" is awkward and unnecessarily includes the possessive "their."
E: Incorrect. Idiom error, as in A. Modifier error: "Due to" must be followed by a noun construction; therefore, (E) is wrong as "due to" is followed by the clause "having the ability of retaining."
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Cacti, like other drought-tolerant plants, can survive with little rain because of having the ability of retaining moisture through modified structures, such as spines and thick stems; however, when there is rain, their wide, shallow root systems allow them to absorb water quickly throughout a large area.

Able to do something is idiomatically better than able of -ing form. So, narrow down to B,C and D. C has pronoun reference error, their can refer to cacti or other plants. Besides, option C and D also distort the meaning of original sentence by removing cause-effect relationship. So, incorrect.

A. rain because of having the ability of retaining - Idiomatically incorrect
B. rain because they are able to retain - Seems no error, hold it.
C. rain, in their ability to retain - Pronoun reference error and this option distorts the meaning also by removing the cause-effect relation of original sentence.
D. rain, having their ability to retain - This option distorts the meaning also by removing the cause-effect relation of original sentence.
E. rain, due to having the ability of retaining - Idiomatically incorrect

Imo. B
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Cacti, like other drought-tolerant plants, can survive with little rain because of having the ability of retaining moisture through modified structures, such as spines and thick stems; however, when there is rain, their wide, shallow root systems allow them to absorb water quickly throughout a large area.

Ability to do something is the correct form

A. rain because of having the ability of retaining
having the ability of -> is unnecessarily wordy. I can do something or i am able to do something. I have the ability to learn quickly. I am having the ability of is wordy. this expression of having can clearly be replaced with simpler base verb forms.

B. rain because they are able to retain
because specifies the reason why they are able to survive. Is Cacti singular or plural? Cacti is plural. Cactus is singular. If confused, check the non-underlined part, it has 'their'...'them' that suggests it is a plural word. so 'they' refers to Cacti. 'Able to' is correctly used. Keep the answer!

C. rain, in their ability to retain
this option distorts the meaning. cacti can survive in little rain in their ability , is it trying to say they can survive in their ability to retain? doesn't make sense!

D. rain, having their ability to retain
Having their ability is used as a modifier to modify Cacti, if we restructure the sentence, Cacti having their ability to retain can survive. there is no need for modifier to represent Cacti.

E. rain, due to having the ability of retaining
based on the meaning, we are looking for reason to explain why Cacti can survive. we want the reason to explain the action of surviving. hence, because is appropriate for this construction. Also ability to is the correct form.

Official Explanation:

Cacti, like other drought-tolerant plants, can survive with little rain because of having the ability of retaining moisture through modified structures, such as spines and thick stems; however, when there is rain, their wide, shallow root systems allow them to absorb water quickly throughout a large area.

A. rain because of having the ability of retaining
B. rain because they are able to retain
C. rain, in their ability to retain
D. rain, having their ability to retain
E. rain, due to having the ability of retaining

A review of our choices reveals a split between the forms of “retain.” We have the infinitive (to retain) and a preposition + -ing word. There is also a split between the use of “ability” or “able to.”

Every answer choice has either “ability” or “able” in the sentence, and these two words involve idioms that have very specific rules. They must always be followed by an infinitive. An infinitive is the word “to” + the base form of a verb. “Ability” and “able” are never followed by any other preposition, like of. So, Options A and E cannot be the correct answers.

Now, we need to consider our sentence as a whole, and because it is a modifying phrase, the part enclosed in commas can be safely ignored for a while. “Cacti can survive with little rain . . . they are able/their ability to retain moisture.” This sentence starts with mentioning how cacti can survive with little water and then mentions their ability to retain moisture. The sentence is attempting to set up a cause and effect statement. “In their ability to retain” and “having their ability to retain” does not show this causal relationship and, instead, seem to merely make a comment about their ability to retain moisture as if it is separate from their surviving with little rain. “Because” does show the causal relationship. Options C and D cannot be the best answer.

Option B is the best answer.
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Well done, sampriya! You cracked the code on this one! Part of why option C is so awkward is because "in their ability to maintain" doesn't show the proper cause-effect relationship. Using "because" fixes this in option B.

Kudos to you!
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Well done mykrasovski! You cracked the code!

I'm curious to know more about how you got to the correct choice using verbs from the non-underlined part of the sentence as your guide. This wasn't really the focus of the question's official explanation, but it's always great to hear how other people tackle these SC questions!
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Great job, HighModeOn! You cracked the code on this one! It would be nice to know more about what you mean when you say options A, D, & E are wrong because of "having." What type of error is that? It may help others who are reading over the comment section to know your thinking process on each of these.
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Thanks for the added information, mykrasovski! We always like to hear the different ways people tackle these questions.
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