A bit confuse with the correct answer stated at the back is B instead of D.

The statement “more power is needed to maintain level flight with a reduced indicated airspeed” sounds like telling me that the aircraft is travelling in the reverse command region which should be the region behind the minimum total drag isn’t?

However, on the other hand I do understand that outside the maximum endurance speed, we are using more power anyway, so it sounds like both of them are correct to me……

Try to keep endurance (power) and range (drag or thrust) separate in your thinking.

For a light piston aircraft, endurance relates to fuel flow which is tied up with power settings.

If you are flying along and the speed reduces, and if you are below the maximum endurance speed, you will be in an unstable situation and the speed will continue to reduce and/or you will descend ... unless you increase power. Now that will also increase thrust which provides for some acceleration to get you back to (or keep you where) you were.