Concept:The nucleus of an atom contains protons and neutrons. Together they are called nucleons.Nucleon number (mass number) A = number of protons Z + number of neutrons N.Formula used:N=A−ZGiven:Nucleon number A=20Number of protons Z=9Solution:Put the values into N=A−Z:N=20−9=11Check: 9 protons +11 neutrons =20 nucleons, which matches the given nucleon number.Key point: Option A (9) is the proton number, not the neutron number. Subtract the protons from the nucleon number.Answer:Option C (11)