They couldn't hear me,.........?(question tag)

Dear student,

Here is the answer:

They couldn't hear me,could they?

Explanation:

If the statement is positive, the question tag is negative. If the statement is negative, the question tag is positive.
For example:

• She is going to school, isn't she? (negative question tag)
• They have not seen him since yesterday, have they? (positive question tag)

In the above sentence since the statement is negative we have used a positive question tag. 

Regards