Which situation could decrease the carrying capacity for humans in a certain

region?

The correct option is 'a crop disease becoming resistant to medicine.'

Explanation:

The maximum number of organisms of a certain species that a habitat can hold is termed as the carrying capacity of that area. Beyond the carrying capacity, the resources will fall short and the organisms will begin to compete for resources.

If a certain crop becomes resistant to a medicine, this will mean that the crop will carry certain disease-causing factors in it. As a result, the crops might die due to the disease or they would not be a source of food for the humans. Hence, the carrying capacity would decrease as a resource is falling short.