Answer:
The correct answer is 5s
Explanation:
Strontium atomic number = 38
Electronic configuration is [tex]$1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 5s^{2}[/tex]
As [tex]$5s$[/tex] orbital is valence orbital in strontium, the outer electrons exists in that orbital only.
