Answer:
B. 6
Step-by-step explanation:
The atomic number of an atom is the number of protons in the nucleus of that atom. So, for example, if an atom has 15 protons in its nucleus, its atomic number is 15.
Protons are positively charged. Since this atom's nucleus contains 6 particles that are positively charged, we can infer that those particles are protons, and that would mean that the atom has 6 protons. Like I said earlier, the number of protons in an atom is the atomic number, so this atom has an atomic number of 6.
Hopefully that's helpful! :)