Answer:
A fundamental theory that provides a description of the physical properties of nature at the scale of atoms and subatomic particles.
Explanation:
A) 
The total energy of the system is equal to the maximum elastic potential energy, that is achieved when the displacement is equal to the amplitude (x=A):
(1)
where k is the spring constant.
The total energy, which is conserved, at any other point of the motion is the sum of elastic potential energy and kinetic energy:
(2)
where x is the displacement, m the mass, and v the speed.
We want to know the displacement x at which the elastic potential energy is 1/3 of the kinetic energy:

Using (2) we can rewrite this as

And using (1), we find

Substituting
into the last equation, we find the value of x:

B) 
In this case, the kinetic energy is 1/10 of the total energy:

Since we have

we can write

And so we find:

The answer is C) an electromagnetic wave
An electromagnetic wave, which includes electromagnetic radiation such as visible light, moves the fastest of all of the options listed by a significant margin, especially through space. In fact, light travelling through space is technically the theoretical limit of how fast something can travel.
Using the Fundamental Equation of Wave, we have:
Letter B
Answer:
The momentum before is equal to the momentum after
Explanation:
It is equal and should level out in an equation.