Explanation:

Valence Electrons: Valence electrons are the electrons present in the outermost energy level (shell) of an atom. These electrons are involved in the formation of chemical bonds and determine the chemical properties of an element.

Groups in the Periodic Table: The periodic table is divided into groups or families based on the number of valence electrons an element has. Elements in the same group have the same number of valence electrons and similar chemical properties.

Exception with Helium (He): Helium is an exception to the general trend because it has a unique electronic configuration and does not follow the pattern of the other elements in its group.

Helium (He) Electronic Configuration: The electronic configuration of helium is 1s², which means it has a total of 2 electrons. The first energy level (shell) can accommodate a maximum of 2 electrons.

Other Group 18 Elements: The other elements in Group 18 of the periodic table are known as noble gases. They have completely filled valence electron shells and are chemically inert. Their electronic configurations are as follows:
- Neon (Ne): 1s² 2s² 2p⁶ (8 valence electrons)
- Argon (Ar): 1s² 2s² 2p⁶ 3s² 3p⁶ (8 valence electrons)
- Krypton (Kr): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ (8 valence electrons)
- Xenon (Xe): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ (8 valence electrons)

Key Point: The key point to note is that helium (He) has only 2 valence electrons, which is different from the other Group 18 elements that have 8 valence electrons. This difference in valence electron configuration is the reason why helium is placed separately in its own group (Group 2) rather than with the other noble gases in Group 18.

Therefore, the correct answer is option D) Helium (He).