Refer to figure.
The Height-Velocity (H-V) envelope for a single engine helicopter it defines a combination of airspeed and height above the ground from which the helicopter will probably not be able to enter autorotation and a safe landing will not be assured.

It is often referred to as “Dead Man’s Curve” and simple helicopters tend to have a fixed H-V diagram, like the figure’s one.

Operations at any height or speed within the shaded area are likely to result in a crash landing:

• In the lower shaded area, in the event of the engine failure, there is insufficient height for the pilot to identify the failure, make the appropriate control inputs and for the pitch attitude to change sufficiently before the helicopter impacts the ground. At low speeds, this is not dangerous, because a running landing results, but at anything above 24 kt a roll-over is likely.
• In the upper area, there is insufficient time to enter autorotation, recover NR and then perform the landing manoeuvre. At higher speeds, the kinetic energy can be quickly converted to NR by flaring, but at low speeds much more height is needed, which provides time to lower the collective and then turn to recover NR before carrying out a landing.

Therefore, only Point D, being outside shaded areas, represents a combination of airspeed and height from which a successful autorotation is most likely to be achieved after an engine failure.