A set of atom in an excited state decays

A set of atoms in an excited state decays.

A set of atoms in an excited state decays

A set of atoms in an excited state decays

which must represent an atom in an excited state?

which must represent an atom in an excited state?

which must represent an atom in an excited state?

What is an excited state of an atom ?

An electron is at ground state of the H atom. The minimum energy required to excite the H atom into the second excited state is