A pull factor refers to a factor that attracts people to move to a certain area. In the context of the examples provided, none of them are traditional pull factors, but given the options, 2. a large deer population could be considered a pull factor, as it might attract people interested in hunting or wildlife.
The other options—volcanic eruption, long period with no rain, and lack of building materials—are more likely to be reasons people leave an area, making them push factors.