We already know what murky means. Hence we infer that the antonym should mean the other of the description of murky, that is, bright, pleasant, and not difficult to work out through. to spot the correct antonym will observe all the options given to us.

Upright means always following the right principles or maintaining a straight body posture. We observe that it doesn't match the description of the antonym we have inferred. Hence, option A is an incorrect answer.

Smoky is synonymous with murky or are some things that smells of smoke. We observe that it doesn't match the description of the antonym we have inferred. Hence, option B is an incorrect answer.

Miserable means being very upset and uncomfortable. We observe that it doesn't match the description of the antonym we have inferred. Hence, option C is wrong .

Luminous means very bright, clear, and gleaming. We observe that it matches the outline of the antonym we have inferred. Hence, option D is that the correct answer.