I'm pretty sure it's A, but then again I'm only in 8th grade. Sorry if this doesn't help:(
The mass is the total number of protons and neutrons there are in the nucleus. There are 15 and 18 neutrons, that means the mass is 33
15 + 18 = 33
Is molecule an answer? It wouldn't let me see all the answers. if it is, molecule is the correct answer. they are held together by a covalent bond, I believe.
