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 are advantageous or disadvantageous. Genetic variation within a population is crucial for natural selection as it provides the raw material. An individual has a fixed set of chromosomes and genetic variation (barring mutations which are rare in an individual's lifespan). A population's genetic variation can be influenced by environmental factors (e.g., a harsh environment may reduce the number of individuals with certain traits, thus altering the genetic variation in the population). The number of chromosomes in an individual or population is species - specific (for a given species, the chromosome number is relatively constant in normal individuals and populations).

Answer:

the amount of genetic variation in a population.