The scenario that would most likely reduce the number of inheritable traits in a population, causing the individuals in that population to remain very similar to one another, is:
A forest that is being converted into farmland.
This is because converting a forest into farmland often involves clearing the land of diverse plant and animal life and replacing it with a limited number of crops, leading to a decrease in biodiversity. This reduced diversity can result in a more homogeneous population of organisms, making them more similar to one another. In contrast, the other scenarios may affect the local ecosystem or individual species, but they do not necessarily lead to the same level of uniformity in traits as agricultural monoculture would.