This is an example of a Type 2 conditional sentence — expressing a hypothetical or imaginary situation in the present or future.
Structure: If + subject + were to + verb, subject + would + verb
Only option (D) follows this structure correctly:
"If he were to win the prize, I would be very happy."
Let's examine the incorrect ones:
(A) Mixes present tense with "would", which is grammatically inconsistent.
(B) Incorrectly pairs "were" (hypothetical) with "will" (real future).
(C) "Was" should be replaced with "were" in unreal conditionals.