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Answer:
The 70%-80% of Max heart rate
Why? will the first two examples (2-3times a week, 15 minutes) aren't really frequency rather it is showing how Frequent the task is done. Cardio is also not a frequency because it is the TASK. Eliminating those and you have a solid answer.
Why the heart rate though? well. the heart rate is obviously the measurement upon the frequency of the heart rate and how much the heart the can actually take.