Which antigen occurs in approximately 20% of individuals of African ethnicity?

The correct answer is Jsa, which is indeed found in approximately 20% of individuals of African descent. Jsa is part of the Kidd blood group system, and its prevalence varies significantly among different ethnic groups. This antigen is particularly associated with individuals of African ancestry, contributing to its higher frequency within that population compared to others.

To provide context regarding the other options, K (Kell antigen) is more prevalent in Caucasians and less so in Africans. Kpa and Kpb (also Kell-system antigens) have varying frequencies, but they are less associated with the specific demographic mentioned in the question. Thus, Jsa’s prevalence is distinctive within the African ethnicity, making it the most fitting answer in this scenario.