The scenario that would most likely reduce the number of inheritable traits in a population, leading to individuals in that population remaining very similar to one another, is:
A swamp that is being drained in order to build houses.
This scenario implies a loss of habitat and biodiversity, which can lead to a decline in the population size of various species. When populations become smaller and more isolated, they are more likely to experience inbreeding and a reduction in genetic diversity, resulting in a population that is less varied and more similar in terms of inheritable traits.