Multiple additions of new genes or alleles to the new population and their loss from the old population of a particular place due to migration is known as:

Gene flow

The correct answer is Option (1)- Gene flow

When migration of a section of population to another place and population occurs, gene frequencies change in the original as well as in the new population. New genes/alleles are added to the new population and these are lost from the old population. There would be a gene flow if this gene migration, happens multiple times.