OFFICIAL EXPLANATION
Over time, weightlessness causes medical problems in astronauts, such as the deterioration of the spinal cord, so astronauts are advised to train rigorously in order to avoid it. A. Over time, weightlessness causes medical problems in astronauts, such as the deterioration of the spinal cord, so astronauts are advised to train rigorously in order to avoid it
Although this answer choice is stylistically flawed as the pronoun it is ambiguous, it is the best of the five answer choices.
The three answer choices that correct the stylistic mistake, namely, the ambiguity, are grammatically incorrect or illogical.
Remember that unlike grammatical mistakes, which must always be corrected, stylistic mistakes, namely, redundancy and ambiguity, are not always corrected.
Therefore, If you don't find a perfect answer choice and have to choose between a grammatically incorrect answer and a stylistically flawed one, choose the latter. B. Over a long period of time, weightlessness causes medical problems in astronauts, such as the deterioration of the spinal cord, so astronauts are advised to train rigorously in order to avoid it
This answer choice is redundant as a result of adding the words a long period of. These words also change the meaning of the original sentence.
C. Over time, weightlessness cause medical problems in astronauts, such as the deterioration of the spinal cord, so astronauts are advised to train rigorously in order to avoid such backbone deterioration
While this answer choice corrects the original ambiguity mistake, it is grammatically incorrect. Replacing the singular verb causes with the plural cause creates a Subject Verb Agreement mistake.D. Over time, weightlessness causes medical problems in astronauts, such as the deterioration of the spinal cord, so astronaut are advised to train rigorously in order to avoid such backbone deterioration
While this answer choice corrects the original ambiguity mistake, it is grammatically incorrect. Replacing the plural noun astronauts with the singular astronaut creates a Subject Verb Agreement mistake. E. Over time, weightlessness causes medical problems in astronauts, such as the deterioration of the spinal cord, but astronauts are advised to train rigorously in order to avoid such backbone deterioration
While this answer choice corrects the original ambiguity mistake, it is illogical as a result of replacing the conclusion word so with the opposition word but.