The electron configuration for a chlorine atom is: 1s² 2s² 2p⁶ 3s² 3p⁵. What electron configuration represents the electrons that make up only the core electrons for a chlorine atom?
a) 1s² 2s²
b) 2s² 2p⁶
c) 3s² 3p⁵
d) 1s² 2s² 2p⁶ 3s² 3p³