The correct answer is: don't they?

• Rule: When the main clause is positive, the question tag is negative.
• Application: The main clause "The scuba divers need air cylinders" is positive.
• Question Tag Formation: To form the question tag, we use the auxiliary verb "do" (since "need" is the main verb) and the subject pronoun "they." To make it negative, we add "don't."

Therefore, the correct question tag is "don't they?"

• Option 1 (“do they?”) is incorrect because a positive statement requires a negative question tag. Since the sentence is affirmative (“The scuba divers need air cylinders”), the tag should be negative.
• Options 3 and 4 (“need they?” / “needn’t they?”) are also incorrect because, in this sentence, “need” is functioning as a main verb, not as a modal auxiliary verb. When “need” is used as a main verb, question tags are formed using do/does/did support.