To determine which electron configuration represents a stable atom, we need to look for configurations that correspond to filled (and thus more stable) electron shells or subshells.
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a. [Ne] 3s² 3p⁵: This configuration represents an atom of chlorine (Cl), which is one electron short of a filled p subshell (which would be 3p⁶). It is relatively stable but not fully stable.
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b. [He] 2s² 2p³: This configuration represents an atom of nitrogen (N) with half-filled 2p subshell. While half-filled subshells can add some stability, it does not represent a full outer shell.
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c. [Ne] 3s² 3p⁶: This configuration represents an atom of argon (Ar), which has a completely filled outer shell (the 3s and 3p subshells). This configuration is stable.
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d. [Ne] 3s² 3p⁶ 4s² 3d⁵: This configuration represents an atom of manganese (Mn). Despite having a filled 3p subshell and additional electrons in the 4s and 3d subshells, it is not fully stable in the context of noble gas configurations. Elements like Mn have special stability but are not completely filled outer shells.
Therefore, the only truly stable electron configuration in the provided options is:
c. [Ne] 3s² 3p⁶.