Which of the following compounds show colour due to d–d transition?

$d \rightarrow d$ transitions require partially filled d orbitals ($d^1$–$d^9$)

| Compound | Metal | O.S. | d config | Colour origin | |----------|-------|------|----------|---------------| | $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ | Cu | +2 | $3d^9$ | $d \rightarrow d$ transition ✓ | | $\text{K}_2\text{Cr}_2\text{O}_7$ | Cr | +6 | $3d^0$ | Charge transfer ✗ | | $\text{K}_2\text{CrO}_4$ | Cr | +6 | $3d^0$ | Charge transfer ✗ | | $\text{KMnO}_4$ | Mn | +7 | $3d^0$ | Charge transfer ✗ |

$\text{Cu}^{2+}$ ($3d^9$) has one unpaired electron and shows $d \rightarrow d$ transition — responsible for the blue colour of $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$.

Answer: Option (1) — $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$