The correct answer is: A. The gain of the system should be decreased.
An unstable system is one that will diverge from its desired output over time. This can be caused by a number of factors, including too much gain, too many poles, or too few zeros. To make an unstable system stable, we need to reduce the amount of gain in the system. This can be done by decreasing the gain of the individual components in the system, or by adding negative feedback to the system.
Option B is incorrect because increasing the gain of an unstable system will only make it more unstable. Option C is incorrect because increasing the number of poles in a system will also make it more unstable. Option D is incorrect because increasing the number of zeros in a system will not necessarily make it more stable.
In conclusion, the correct answer is A. The gain of the system should be decreased.