[Solution from source] 1.73 BM = √(n(n + 2)) with n = 1 unpaired electron. Cu²⁺ (3d⁹) in [Cu(NH₃)₄]²⁺ has one unpaired electron. [Ni(CN)₄]²⁻ is square planar and diamagnetic, TiCl₄ has Ti⁴⁺ (3d⁰), and [CoCl₆]⁴⁻ has high-spin Co²⁺ (3d⁷) with three unpaired electrons. Answer (1).