#### Answer

(a) Na
(b) F
(c) O

#### Work Step by Step

(a) As given that there are 11 protons and 12 neutrons so atomic number is 11 and mass number is 23 which means that it is sodium denoted by the symbol $Na$
(b) As given that there are 9 protons and 10 neutrons so atomic number is 9 and mass number is 19 which means that it is Fluorine denoted by the symbol $F$
(c) As given that there are 8 protons and 8 neutrons so atomic number is 8 and mass number is 16 which means that it is Oxygen denoted by the symbol $O$