For \([V(\text{H}_2\text{O})_6]^{3+}:\)
Vanadium (V) has an atomic number of 23, with an electronic configuration of \([ \text{Ar} ] 3d^3 4s^2\). \(\text{V}^{3+}\) configuration: \([ \text{Ar} ] 3d^2\). Number of unpaired \(d\)-electrons: 2 (even number).
For \([Cr(\text{H}_2\text{O})_6]^{2+}:\)
Chromium (Cr) has an atomic number of 24, with an electronic configuration of \([ \text{Ar} ] 3d^5 4s^1\). \(\text{Cr}^{2+}\) configuration: \([ \text{Ar} ] 3d^4\). Number of unpaired \(d\)-electrons: 4 (even number).
For \([Fe(\text{H}_2\text{O})_6]^{3+}:\)
Iron (Fe) has an atomic number of 26, with an electronic configuration of \([ \text{Ar} ] 3d^6 4s^2\). \(\text{Fe}^{3+}\) configuration: \([ \text{Ar} ] 3d^5\). Number of unpaired \(d\)-electrons: 5 (odd number).
For \([Ni(\text{H}_2\text{O})_6]^{3+}:\)
Nickel (Ni) has an atomic number of 28, with an electronic configuration of \([ \text{Ar} ] 3d^8 4s^2\). \(\text{Ni}^{3+}\) configuration: \([ \text{Ar} ] 3d^7\). Number of unpaired \(d\)-electrons: 3 (odd number).
For \([Cu(\text{H}_2\text{O})_6]^{2+}:\)
Copper (Cu) has an atomic number of 29, with an electronic configuration of \([ \text{Ar} ] 3d^{10} 4s^1\). \(\text{Cu}^{2+}\) configuration: \([ \text{Ar} ] 3d^9\). Number of unpaired \(d\)-electrons: 1 (odd number).
From the analysis above, only \([V(\text{H}_2\text{O})_6]^{3+}\) and \([Cr(\text{H}_2\text{O})_6]^{2+}\) have an even number of unpaired \(d\)-electrons.
The number of complexes with an even number of unpaired \(d\)-electrons is 2, corresponding to Option (1).