The valence electrons in an atom of phosphorus in the ground state are all found in IN THE THIRD SHELL.
Phosphorus has an atomic number of 15, therefore its electronic configuration is 2, 8, 5. This configuration implies that, phosphorus electrons are distributed in three different shells and the number of electrons in the outermost shell, which is the third shell, is 5. The ground state refers to a state, where the atoms of phosphorus are not excited. Electrons move away from their ground states when they are excited.