What is thermal energy?
a. thermal energy is a measure of a substance specific heat
b. thermal energy is the measure of movement of the substances molecules
c. thermal energy is the same as the substances temperature
d. thermal energy is measure of a substance vaporization

Heat comes from the movement of molecules so it's B

Base SI unit is the unit that used for simple quantity like time=second and length= meter. They only have one unit.
Derived unit is more complex because you multiply or divide at least two base SI, making it have more than 1 unit. The example could be velocity which was time/length = m/s