Pretty Sure its C) a decrease in resting heart rate this is because when you exercise your heart beats faster during the activity but exercising everyday, in time usually slows your resting heart rate.
Couldn't be (A) because if you're improving fitness you would need more sleep.
Couldn't be (B) because if you improve your fitness you would eat more calories.
Couldn't be (D) because if when you improve fitness you would essentially gain weight by building muscle.