The correct option is C nonetheless
The given sentence expresses two ideas in contrast. On one hand the speaker loves the color orange, while on the other hand, the dress in orange seems gaudy to the speaker. Hence an adverb is required that refers to contrast.
In option A, 'nonetheless', refers to contrast, it means 'in spite of that', hence it is the most suitable option.
In option B, 'therefore' refers to inference or conclusion; it is used when one sentence is the conclusion of another. for example, 'I love orange therefore I bought this orange dress.' It isn't relevant here, hence B is incorrect.
In option C, 'if' refers to condition, or a hypothetical situation. Here the sentence isn't conditional, it's adversative (expressing contrast). Hence C is incorrect.
In option D, 'moreover' refers to an addition, it is used when the sentences do not affect each other. But here the sentences share a relation of contrast, hence D is incorrect.