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.
Draining a swamp can lead to habitat destruction and loss of biodiversity, which can reduce the genetic diversity of the species that inhabit that swamp. Over time, this can result in a population with fewer inheritable traits as the remaining individuals are subjected to more uniform environmental pressures, leading to similar adaptations and traits within the population.
The other scenarios, while they can also affect biodiversity, the swamp's drainage is particularly impactful in terms of immediate habitat loss and the potential for decreased genetic variation in a specific population.