Step 1: Orbital splitting:
Ligands split the d orbitals by an energy gap $\Delta$, which for many complexes matches visible light.
Step 2: Absorption:
An electron absorbs a photon with energy equal to $\Delta$ and moves up, so the compound shows the complementary colour.
Step 3: Check with exceptions:
$d^0$ and $d^{10}$ ions have no d-d jump and are colourless. So the cause is d-d transitions, option (A).
Final Answer:
Colour arises from d-d transitions.
\[ \boxed{\text{(A) }d\text{-}d\ \text{transitions}} \]