The scenario that would most likely reduce the number of inheritable traits in a population, causing individuals to remain very similar to one another, is:
A swamp that is being drained in order to build houses.
This scenario can significantly decrease habitat diversity and the overall population size of the organisms living in the swamp. As the swamp is drained and habitat is lost, a smaller population remains, which can lead to a reduction in genetic diversity due to inbreeding. Inbreeding reduces the variability of inheritable traits, making the individuals in the population more similar to each other.
The other options also involve habitat changes but may not have the same immediate impact on genetic diversity as the draining of a swamp would.