The correct answer is: don't they?

Explanation:

• 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?"