The correct option is A frequency
Based on experiments on Drosophila, Morgan suggested that the frequency of crossing over between two genes would depend on the physical distance between them.
Alfred Sturtevant, who was T.H. Morgan’s student, calculated the crossover percentage of two genes. This crossover percentage is called as frequency of recombination between gene pairs on the same chromosome. He used the frequency of recombination, also known as crossover value between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome (genetic maps).