If we throw a light thing from a height ir dosent reaches so faar and faast but if we throw a hevier thng from a height ir reaches far and fast too. why is it so?

It is only due to air resistance ! if air resistance is not there , both will acquire same speed and fall with same speed !!

If you did the same experiment where there is no matter or simply we say 'vacuum', you'll see that no matter what their mass is they'll fall at same speed and will reach ground at the same time, whereas this case is completely different practically. 
The reason behind is 'drag force' or 'air resistance'.

More the matter is less will be drag force and less the matter is more will be the drag force.
This drag force reduces the gravitational force exerted by earth and hence its downward acceleration decreases. As a result lighter object takes more time to reach ground than heavier objects.
Hope this helps!

The duration of a photographic flash is related to an rc time constantly

Ibelieve the answer is b