C. Number of protons = atomic number; number of neutrons = mass number - atomic number
Right, as you mentioned in the comments, you find 
by plugging in the different values of 
.
For 
, we have
Similarly, for 
, you get
The easiest way to build a unit for energy is to remember that
'work' is energy, and
Work = (force) x (distance).
So energy is (unit of force) x (unit of distance)
[Energy] = (Newton) (meter) .
'Newton' itself is a combination of base units, so
energy is really
(kilogram-meter/sec²) (meter)
= kilogram-meter² / sec² .
That unit is so complicated that it's been given a special,
shorter name:
Joule .
It doesn't matter what kind of energy you're talking about.
Kinetic, potential, nuclear, electromagnetic, food, chemical,
muscle, wind, solar, steam ... they all boil down to Joules.
And if you generate, use, transfer, or consume 1 Joule of
energy every second, then we say that the 'power' is '1 watt'.
