Final answer:
Concordance is used when studying genetic linkage and analysing recombination frequencies between genes, while heritability assesses the genetic contribution to phenotypic variance in a population.
Step-by-step explanation:
Concordance should be used instead of heritability when studying genetic linkage (option a), as it focuses on whether different traits are inherited together due to the genes being physically linked on the same chromosome. Concordance is particularly relevant when analysing the frequencies of recombination between genes to determine if they show independent assortment or are linked. In contrast, heritability assesses the proportion of phenotypic variance in a population that is attributable to genetic variance.