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 swamp that is being drained in order to build houses.
This scenario often leads to habitat loss and fragmentation, which can decrease the genetic diversity of the organisms that inhabit the swamp. As habitats become reduced or isolated, populations may experience inbreeding, which can lead to a reduction in genetic variability. In contrast, the other scenarios may have different ecological impacts but do not necessarily lead to such a pronounced reduction in genetic diversity as habitat destruction and fragmentation do.