To best meet the goal of adding one anaerobic workout to his weekly schedule, Marcus should choose HIIT (High-Intensity Interval Training). HIIT is specifically designed to involve short bursts of intense exercise followed by rest or lower-intensity periods, which makes it a highly effective anaerobic workout.
Swimming and running are primarily aerobic exercises, while Zumba can be a mix but is generally more aerobic-focused as well. Therefore, HIIT would be the most suitable choice for anaerobic training.