Those conditions are met when you have an elastic collision of equal masses.
So the answer would be a tennis ball.
Let us write momentum conservation law for this problem. The left-hand side is before the collision and right-hand side is after the collision:
What this equation means is that all momentum of the first ball is transferred to the second ball and velocities before and after the collision are the same.
There is no need to write energy conservation law. We can see that above equation holds true only if

.
Whatever depends on the voltage would change very quickly i guess
You need to know everything is made of atoms they are always colliding and they are never still because you'd think that every thing in space is still all the time even if it's not making visibly
The water leval in the tub will stay the same due to the law of water replacement.
(a) 
The energy of a photon is given by:

where
is the Planck constant
is the speed of light
is the wavelength
For the microwave photon,

So the energy is

And converting into electronvolts,

(b) 
For the energy of the photon, we can use the same formula:

For the visible light photon,

So the energy is

And converting into electronvolts,

(c) 
For the energy of the photon, we can use the same formula:

For the x-ray photon,

So the energy is

And converting into electronvolts,
