Answer is: 1s22s22p5.
1) Electron configuration 1s²1p⁶2d², this is not reasonable because 1p and 2d oritals do not exist.
2) Electron configuration 1s²2s⁴2p⁶, this is not reasonable because s orbitals only contain 2 electrons.
3) Electron configuration 1s²2s²2p⁵ is of an element fluorine. Fluorine (F) has atomic number 9, which means it has 9 protons and 9 electrons.
4) Electron configuration 1s²2s²2d⁶, this is not reasonable because 2d orbitals do not exist.