Answer:
The environmental factor that could lead to a decrease in genetic variation in a tuna population is an increase in pollution (second option).
Step-by-step explanation:
There is a correlation between genetic variability and environmental pollution, the latter being a factor that impacts negatively on the variability of a specific population.
The concept of pollution stress not only implies a low rate of reproduction, but it is also a factor that prevents genetic exchange with other populations, which is a factor that makes the genetic variability decrease in a population.
For these reasons an increase in pollution implies a decrease in genetic variability in a tuna population.