Answer: The correct answer is "the speed of the wave becomes four times".
Explanation:
The relation between the speed, frequency and the wavelength is as follows:
v=f\lambda
Here, v is the speed of the wave, f is the frequency and \lambda is the wavelength.
The speed of the sound wave is directly proportional to the frequency.
In the given problem, if the speed of the sound wave is increased four times then the speed of the sound becomes four times.
Therefore, the speed of the sound wave becomes four times.
Answer:

direction is Horizontal
Explanation:
As we know that the string is horizontal here
so the tension force in the string is due to electrostatic force on it
now we will have

so here the force is tension force on it


now we have


direction is Horizontal
Energy can be one answer! There are many, but energy is a main one.
A)Ep'=mgh=mgl(1-cosa).At the bottom of the swing Ep=0(reference level),so the potential energy as the child is just released is bigger than the potential energy at the bottom of the swing.;B)The speed of the child at the bottom of the swing-->v=√(2gh)=√[2gl(1-cosa)];C)I don't think that the tension does any work.