Question

environmental factors can influence natural selection because they can increase or decrease
the number of chromosomes in an individual
the number of chromosomes in a population
the amount of genetic variation in an individual
the amount of genetic variation in a population.

Explanation:

Natural selection acts on populations. Environmental factors can affect which traits (and thus which genetic variations) are favorable or unfavorable. This influences the amount of genetic variation in a population over time. The number of chromosomes in an individual or population is typically stable (except in cases of rare mutations like polyploidy which is not the norm here). Genetic variation in an individual is mainly due to processes like mutation and meiosis, but natural selection (influenced by environment) acts on the population - level genetic variation.

Answer:

the amount of genetic variation in a population.