1. Reason for colour in lanthanide ions
   Lanthanide ions show colour due to $f$-$f$ electronic transitions.
   These transitions are possible only when the ion has partially filled $4f$ orbitals.

2. Check electronic configurations of the given ions

   • $La^{3+}$
     $La$ has atomic number $57$.
     Neutral configuration: $[Xe]5d^16s^2$
     $La^{3+} = [Xe]4f^0$ No $4f$ electron  no $f$-$f$ transition  colourless.

   • $Sm^{3+}$
     $Sm$ has atomic number $62$.
     Neutral configuration: $[Xe]4f^66s^2$
     $Sm^{3+} = [Xe]4f^5$ Partially filled $4f$ subshell  $f$-$f$ transitions possible  coloured.

   • $Gd^{3+}$
     $Gd$ has atomic number $64$.
     Neutral configuration: $[Xe]4f^75d^16s^2$
     $Gd^{3+} = [Xe]4f^7$ This is a half-filled $4f$ subshell. Although such ions are relatively less intensely coloured, they still have partially filled $4f$ orbitals and can show colour.

   • $Lu^{3+}$
     $Lu$ has atomic number $71$.
     Neutral configuration: $[Xe]4f^{14}5d^16s^2$
     $Lu^{3+} = [Xe]4f^{14}$ Completely filled $4f$ subshell  no $f$-$f$ transition  colourless.

3. Evaluate options

   • A: $Sm^{3+}$  coloured
   • B: $Gd^{3+}$  also coloured
   • C: $Lu^{3+}$  colourless
   • D: $La^{3+}$  colourless

4. Conclusion
   The ions with partially filled $4f$ orbitals should show colour. Hence both $Sm^{3+} \text{ and } Gd^{3+}$ can show colour.

Since this is given as a single-correct MCQ, the stored answer $A$ is incomplete. Option $B$ should also qualify on the basis of standard lanthanide colour theory.