Official Solution:

The dinosaurs in the late cretaceous period failed to survive on the planet because of factors such as inadequate food supplies, harsh weather, and an inability to adapt to the changing environment.

A. an inability
B. that they were unable
C. because they were unable
D. being unable
E. lacking an ability

1. Correct. The sentence contains a list of factors that kept the dinosaurs from surviving on Earth: "inadequate food supplies, harsh weather, and an inability to adapt to the changing environment." In lists of examples, all the examples must be parallel (in the same form). The list has, as its elements, nouns, which are parallel. Thus, this choice contains no error. If you understand the list, this is a pretty straightforward parallelism question.
2. This choice a clause "that they were unable to communicate with Native Americans" is a part of the list instead of a noun, thus violating the parallelism.
3. As in (B) this choice is incorrect because "and because they were unable to adapt …." is a clause rather than a noun, thus violating the parallelism.
4. This choice is incorrect because "and being unable to ….." is a verb construction rather than a noun, thus violating the parallelism.
5. This choice is incorrect because "and lacking ability to …." is a verb construction rather than a noun, thus violating the parallelism.

Answer: A
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I think this is a high-quality question and the explanation isn't clear enough, please elaborate. Why "an" inability and not "inabilities"?

Hi, The sentence talks about factors because of which dinosaurs failed to survive.
Your observation is correct. All the list items are in structure as adj+Noun, but eventually it is a list of nouns. In choice B "that" modifies the entire clause "they were unable to..." and thus is not correct. In choice A it starts with noun "inability", even if it lacks adj, it still maintains list of nouns and it is correct.

The use of "and" here -
Whenever we have more than 2 elements in a list, We express them as x, y and z. "and" in this case is used in same sense and it is used to maintain the parallelism and not to distort.

Hope it helps..