Answer:
(3) the seventh shell
Explanation:
Let's consider the electron configuration of radium in the ground state.
₈₈Ra: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s²
As we can see, the valence electrons are in the 7s shell, that is, the seventh shell. These are the electrons that participate in bonds.