The scenario that would 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 is because draining a swamp leads to habitat loss and can result in a reduction of genetic diversity due to population bottlenecks, where only a few individuals may survive and reproduce. This can increase the likelihood of inbreeding and decrease the overall variability of traits in the population, leading to individuals being more similar to each other.