<span>the statement that is true regarding flexibility is : b. a joint's range of motion will be lost if the joint is not used regularly.
Our body is like a machine. If we not constantly heat it up, our body will be more prone to injury. We can see that the old people who lived within the tribe in the middle of the mountain are far stronger than the one who lived in the city.</span>
In projectile motion the horizontal component is not affected by gravity. You split the motion in horizontal and vertical motion. The horizontal will be an uniform motion, with v being constant. While the vertical will be an uniform accelerated motion, with the acceleration being the gravity.
1) Acceleration: 
The motion of the plane is a uniformly accelerated motion, so we can find its acceleration by using the suvat equation

where
v is the final velocity
u is the initial velocity
a is the acceleration
s is the displacement
Here we have
v = 150 m/s is the final velocity of the plane
u = 0 (it starts from rest)
a=?
s = 1500 m is the displacement
Solving for a, we find

2. Time: 20 s
For this part of the problem, we can use another suvat equation:

v is the final velocity
u is the initial velocity
a is the acceleration
t is the time
Here we already know:
v = 150 m/s is the final velocity of the plane
u = 0 (it starts from rest)
(found in part 1)
Solving for t, we find the time taken for the plane to reach the final velocity of 150 m/s:

Yes it helps improve flexibility