In which case is potential energy increasing? A.
a 20 kilogram boulder rolls down a hill that is 430 meters high
B.
a weightlifter lifts a 20 kilogram barbell above his head
C.
a skydiver jumps from an airplane that is 4 kilometers above the Earth
D.
a spring of a dart gun launches a dart in a straight line from rest position

a weightlifter lifts a 20 kilogram barbell above his head

I think it's B " Weightlifter lifts a 20 Kg barbell above his head"

after a the power stroke, adp is released, but the cross-bridge formed is still in place. atp then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.