Which of the following examples can cause gene flow between populations?

an increase in predators to a population of zebra

a volcanic eruption that kills half of a population of mice

bird droppings that contain seeds from a different location

a person that sprays a toxin on a group of mosquitoes

The answer is bird droppings that contain seeds from a different location.

Gene flow includes a transfer of alleles from one population to another. So imagine a population of plant A on location 1 and another population of plant A on location 2. They cannot interchange their genes. But if birds drop seeds from location 1 and those seeds get to location 2, gene flow may occur.