Questions: Within a decade of the Montreal Protocol, what did scientists see? The rate of ozone thinning slowed down and stopped, but it did not reverse. The rate of ozone thinning slowed down and then began to increase again. The rate of ozone thinning slowed down and began to reverse. The rate of ozone thinning increased exponentially.

The answer is the third one: The rate of ozone thinning slowed down and began to reverse.

Explanation for each option:

1. The rate of ozone thinning slowed down and stopped, but it did not reverse. - Incorrect. Scientific observations showed that the ozone layer began to recover, not just stop thinning.
2. The rate of ozone thinning slowed down and then began to increase again. - Incorrect. This is contrary to the observed recovery of the ozone layer following the implementation of the Montreal Protocol.
3. The rate of ozone thinning slowed down and began to reverse. - Correct. The Montreal Protocol led to a significant reduction in the emission of ozone-depleting substances, resulting in the gradual recovery of the ozone layer.
4. The rate of ozone thinning increased exponentially. - Incorrect. The Montreal Protocol was successful in reducing the rate of ozone depletion, not increasing it.

Summary: Within a decade of the Montreal Protocol, scientists observed that the rate of ozone thinning slowed down and began to reverse, indicating the effectiveness of the international agreement in addressing ozone depletion.