Answer
See the detailed answer below.
Work Step by Step
$\bullet$ The Ground-State Electron Configuration of Calcium (Ca, $ Z = 20 $) is:
$$
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2
$$
This configuration shows that calcium has a completely filled $ 4s $ subshell with two electrons, and all lower energy subshells are also fully occupied.
$\bullet$ The Ground-State Electron Configuration of Nickel (Ni, $ Z = 28 $) is:
$$
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^8
$$
In this case, nickel has two electrons in the $ 4s $ subshell and eight electrons in the $ 3d $ subshell.
$\bullet$ The Ground-State Electron Configuration of Arsenic (As, $ Z = 33 $) is:
$$
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3
$$
Arsenic has a fully filled $ 3d $ subshell with 10 electrons, a filled $ 4s $ subshell, and three electrons in the $ 4p $ subshell.
$\bullet$ The Ground-State Electron Configuration of Krypton (Kr, $ Z = 36 $) is:
$$
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6
$$
Krypton has a completely filled $ 4p $ subshell with six electrons, along with completely filled lower-energy subshells, making it a noble gas with a stable electron configuration.
See the 4 figures below.
