Population density in a habitat increases due to processes that add individuals:

• Natality (Birth rate): New individuals are produced within the population.

• Immigration: Individuals from other areas enter the population.

Both of these lead to a rise in population density.

Other options include processes that either decrease or do not increase population:

• Mortality reduces population size.

• Emigration leads to individuals leaving the population.

Hence, the correct combination responsible for increasing population density is natality and immigration.