Step 1: Approach
Link colour to the electronic structure.
Step 2: Reasoning
Visible colour needs the absorption of light by a d electron moving to a higher d level. If there are no unpaired electrons (an empty or a full d subshell) there is nothing suitable to excite, so light passes through and the ion is colourless.
Step 3: Examples
$\text{Cu}^{+}$ ($3d^{10}$) and $\text{Ti}^{4+}$ ($3d^{0}$) are colourless. $\text{Cu}^{2+}$ ($3d^{9}$, one unpaired electron) is blue.
Step 4: Conclusion
A colourless ion is predicted to have 0 unpaired electrons, option (A).
Final Answer:
A colourless transition metal ion has no unpaired d electrons, option (A).
\[ \boxed{\text{0 unpaired electrons}} \]