True when the boron control rods are lowered it slows the reaction
You can't usually just use a single spectrum line to confirm the identity of an element because there are cases that the emission line id not clearly defined. When the emission line is very weak compared to surrounding noise, in which case the more datapoints you have to build up confidence for the existence of a particular emission spectra, the better.
A is correct because there is no reaction involved
Create more of their species
The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element.