Explanation:

- "Noxious" means harmful or poisonous.

- Option 1: "Pernicious" means having a harmful effect, especially in a subtle way. It is similar in meaning to "noxious."

- Option 2: "Unwholesome" means detrimental to health or well-being. Like "noxious," it implies something harmful.

- Option 3: "Stubborn" refers to someone or something that is difficult to move or change. It does not relate to the harmful implications of "noxious."

- Option 4: "Harmless" means not able or likely to cause harm. It is the opposite of "noxious."

Option 4: Harmless

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