Were it not for the fusion-powered heat and radiation that rush from its core, instead its own weight would cause a star to collapse.

(A) instead its own weight would cause a star to collapse

(B) instead a star would have collapsed under its own weight

(C) a star would have to be collapsing under its own weight

(D) a star would collapse under its own weight

(E) its own weight would have caused a star’s collapse

Were it not for the fusion-powered heat and radiation that rush from its core, instead its own weight would cause a star to collapse.
A. instead its own weight would cause a star to collapse
B. instead a star would have collapsed under its own weight
C. a star would have to be collapsing under its own weight
D. a star would collapse under its own weight
E. its own weight would have caused a star’s collapse

This is a form of if construction and sets up a hypothetical condition. This construction requires would.

If I were rich I would donate money to charity.

This leaves us with options C&D. would have to be is wrong construction.

Correct option D

2:3 split. the subject is the star so it should be followed. C sounds weird and doesnt make much sense. D
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"its core" modifies the star.. and hence down to C and D..
D is uncomplicated and conveys the meaning straight!