(C) The correct option is $(C)$.For the ion ${}_{19}^{40} K^{+}$:Atomic number $(Z) = 19$,Mass number $(A) = 40$.Number of electrons in neutral $K$ at

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(C) The correct option is $(C)$.For the ion ${}_{19}^{40} K^{+}$:Atomic number $(Z) = 19$,Mass number $(A) = 40$.Number of electrons in neutral $K$ atom $= 19$. Since it is a $K^{+}$ ion,it has lost $1$ electron,so number of electrons $= 19 - 1 = 18$.Number of neutrons $= A - Z = 40 - 19 = 21$.Sum of electrons and neutrons $= 18 + 21 = 39$.