He couldn't decide ______ to buy the red or the blue jacket.

• A. Whether
• B. If
• C. And
• D. Or

Answer: (A)

When you want to show a decision between two options, you use whether to + verb. Here, the speaker is deciding between buying different jackets, so the structure is decide whether to buy the red or the blue jacket. Whether signals the choice between the two possibilities and smoothly links to the infinitive that describes the action being considered.

Using if would feel less formal and is not the standard choice in this pattern. And/Or don’t fit here because they don’t introduce the decision clause in the way needed for choosing between options. The phrase “the red or the blue jacket” remains the two options being weighed, and the main verb is the decision about buying.

So the natural, correct sentence is: He couldn't decide whether to buy the red or the blue jacket.