When the migrated individuals breed within the new population, the immigrants add new alleles to the local gene pool of the host population and is considered as:
Gene pool
Gene variation
Hybridisation
Gene flow
As we have already discussed in Mechanism of Evolution: Gene Mutation & Gene Flow -
The migrated individuals breed within the new population, the immigrants will add new alleles to the local gene pool of the host population. This is called gene flow or gene migration. Hence, the correct option is (d).