Which of the following complexes will exhibit maximum attraction to an applied magnetic field?

Maximum attraction to a magnetic field means maximum paramagnetic character, which requires the maximum number of unpaired electrons.

Option 1: [Ni(H₂O)₆]²⁺ - Ni²⁺ has d⁸ configuration. H₂O is a weak field ligand. In octahedral field: t₂g⁶ eg² → 2 unpaired electrons.

Option 2: [Co(en)₃]³⁺ - Co³⁺ has d⁶ configuration. en is a strong field ligand. In octahedral field: t₂g⁶ eg⁰ → 0 unpaired electrons (diamagnetic).

Option 3: [Zn(H₂O)₆]²⁺ - Zn²⁺ has d¹⁰ configuration → 0 unpaired electrons (diamagnetic).

Option 4: [Co(H₂O)₆]²⁺ - Co²⁺ has d⁷ configuration. H₂O is a weak field ligand. In octahedral field: t₂g⁵ eg² → 3 unpaired electrons.

[Co(H₂O)₆]²⁺ has the maximum number of unpaired electrons (3), so it shows maximum paramagnetic character and maximum attraction to a magnetic field.

This matches option 4.