The scenario that would 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 swamp that is being drained in order to build houses.
This scenario suggests a significant alteration of the habitat, which can lead to reduced genetic diversity due to habitat loss, population bottlenecks, and possibly inbreeding among the remaining individuals. The other scenarios describe environments that may become inhospitable or change, but they do not directly imply the same level of human impact and habitat fragmentation that can drive down genetic diversity as strongly as the draining of a swamp for housing development.