Which structure contains pairs of opposing skeletal muscles

The correct answer would be "The test tube that stops bubbling first contains the catalyst".

Catalyst refers to a substance that increases the rate of reaction without getting consumed in the reaction.

It lowers down the activation energy of the reaction which accelerates the rate of reaction.

Thus, the reaction will be completed the fastest in the test tube which contains the catalyst.

Hence, formation of bubbles will stop earliest in the test tube containing the catalyst.