Which of the following f-block elements in the +3 oxidation state is colourless?

1. $$\mathrm{La^{3+}}$$ 

- Configuration: $$[\mathrm{Xe}]\,4f^0$$

- Reason: It has an empty $$f$$-subshell. Without electrons, no $$f\text{-}f$$ transitions can occur to absorb visible light.

2. $$\mathrm{Gd^{3+}}$$ 

- Configuration: $$[\mathrm{Xe}]\,4f^7$$

- Reason: It has a half-filled $$f$$-subshell. Its electronic transitions absorb mainly in the ultraviolet region, leaving it colourless in the visible spectrum.

3. $$\mathrm{Lu^{3+}}$$

- Configuration: $$[\mathrm{Xe}]\,4f^{14}$$

- Reason: It has a completely filled $$f$$-subshell. There are no vacant orbitals available for $$f\text{-}f$$ electronic transitions.

Correct option: (D)