Final answer:
Positive assortative mating can increase heritability estimates by reducing genetic variation within a population.
Step-by-step explanation:
Positive assortative mating refers to the tendency of individuals to mate with others who are similar to themselves in certain traits. This can lead to an increase in heritability estimates because it increases the likelihood that offspring will inherit similar traits from both parents. By mating with individuals who have similar traits, the genetic variation within a population decreases, making it easier to measure the influence of genes on certain traits. Therefore, the correct answer is a) Increase heritability estimates.