Which point in the graph does a parachute open? 

The diagram represents speed-time for a sky diver. He falls freely
  from an aircraft then opens a parachute and later lands safely on the
  ground. At which point did the parachute fully open?

It's also not drawn quite right at the end. The speed definitely does not increase at any point after B. 
My guess was B but it turned out to be C.
The answer presented:

The sky diver is falling under gravity known as free-falling at the
  beginning. The speed of the sky diver increases with time with
  decreasing acceleration till point B. The speed decreases when the
  parachute opens at point B. The parachute fully opens at point C.
Note: This questions ask (sic) which point did the parachute fully
  opened (sic), not when did the parachute just opened, (sic) which is
  point B.

Questions:


*

*In what way does a parachute not "fully" open?

*Is C necessarily THE point between B and D wherein the parachute fully opens? I was thinking C is merely the closest answer among A, B, C and D.

*Why does the diver speed up at a decreasing rate? Since the acceleration due to gravity is constant, I was thinking that the diver speeds up constantly.
 A: *

*The parachute can not be opened instantaneously.

*The parachute is fully opened at the "vinicity" of the point $C$.

*The acceleration due to gravity is constant but there is another force one needs to take into account : the air resistance. 
A: Air drag is roughly proportional to the square of the velocity, the area of the object, the density of the air, and some "drag factor" that depends on the shape. For turbulent drag, we usually write
$$F = \frac12 \rho v^2 A C_D$$
In the case of the man falling from the plane, he will experience greater drag as his velocity goes up - so instead of linear acceleration, he will accelerate "more slowly" as his speed increases, until he reaches "terminal velocity" where the force of drag is equal to the force of gravity. For a human, this is around 200 km/h (depending on the position of the skydiver, etc).
Now as the parachute is opened, it will initially not have a large area, and not present the largest possible drag; but as it opens more fully (takes on its final shape), the force will increase and the deceleration will be greater. Then, as the man slows down, the drag from the parachute will decrease again.
With this in mind, you would expect the speed curve to have a corner when the parachute is first opened, but then become gradually steeper until the parachute is fully opened (but the speed is still high), then flatten again as the man slows down (with parachute fully deployed):

In this diagram you see that the parachute starts to open at B (corner) and is fully deployed at C (where the slope is steepest).
The graph does not really give you a clear situation of "deceleration getter bigger" like my graph does between B and C - but given that the slope of the given curve is greater at B than at C, I would answer "B". But I was never one for multiple choice... 
