Mass number = atomic number(number of protons) + number of neutrons.
Sodium-20 - 20 is a mass number
Atomic number you need to find from the Periodic table. It is the ordinal number of the element.
Atomic number of Sodium (Na) is 11.
20=11+ number of protons
Number of neutrons is 20-11=9.
Correct answer D.9.