What is the correct definition of intensity?

a.
with regard to physical fitness, the number of days you work out each week.

b.
the number of heartbeats per minute that you should aim for during moderate to vigorous activity to benefit your circulatory system the most.

c.
the number of exercises you do to prepare your muscles for moderate to vigorous activity.

d.
with reference to physical fitness, how much energy you use when you work out.