Final answer:
Option (c), Mutation introduces new alleles, including harmful ones, into a population, while natural selection can decrease the frequency of harmful alleles by favoring traits that are advantageous for survival and reproduction.
Step-by-step explanation:
The correct answer to the question of which evolutionary forces can introduce new harmful alleles into a population, and which will decrease their frequency, is c) Mutation; natural selection. Mutation is a change in the DNA sequence that can introduce new alleles into a population. These mutations can be beneficial, harmful, or neutral.
Harmful mutations may be removed from the population through the process of natural selection, as individuals with these mutations are less likely to survive and reproduce. Natural selection tends to decrease the frequency of harmful alleles over time because it favors the survival and reproduction of individuals with advantageous traits.