Were it not for the fusion-powered heat and radiation that rush from

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Here is a detailed explanation to this question-

Concepts tested here: Grammatical Construction + Idioms + Awkwardness/Redundancy

. "would + base form of verb" is a correct construction for referring to a hypothetical future action.
. “would + have + past participle" construction is used to refer to a hypothetical action in the past.

A: This answer choice fails to form a complete sentence; both "Were it not...core" and "instead its own weight would cause a star to collapse" are dependent clauses, so there is no independent subject for the modifiers to act upon. Further, Option A uses the passive voice construction "its own weight would cause a star to collapse", leading to awkwardness and redundancy.

B: This answer choice fails to form a complete sentence; both "Were it not...core" and "instead a star would have collapsed under its own weight" are dependent clauses, so there is no independent subject for the modifiers to act upon. Further, Option B incorrectly uses the "would + have + past participle ("collapsed" in this sentence)" construction to refer to a hypothetical future action; please remember, "would + base form of verb" is a correct construction for referring to a hypothetical future action, and “would + have + past participle" construction is used to refer to a hypothetical action in the past.

C: This answer choice incorrectly uses the construction "would have to be collapsing" to refer to a hypothetical future action; please remember, "would + base form of verb" is a correct construction for referring to a hypothetical future action.

D: Correct. This answer choice uses the independent clause "a star would collapse under its own weight"; thus, it acts upon the independent noun "a star" with the active verb "would collapse" to form a complete thought, leading to a complete sentence. Further, Option D correctly uses the "would + base form of verb ("collapse" in this sentence)" to refer to a hypothetical future action. Additionally, Option D is free of any awkwardness or redundancy.

E: This answer choice incorrectly uses the "would + have + past participle ("caused in this sentence)" construction to refer to a hypothetical future action; please remember, "would + base form of verb" is a correct construction for referring to a hypothetical future action, and “would + have + past participle" construction is used to refer to a hypothetical action in the past.

Hence, D is the best answer choice.

All the best!
Experts' Global Team