Correct Answer: V²⁺

Magnetic moment (spin only) is given by:

μ = √[ n(n + 2) ] B.M.

Given μ = 3.87 B.M.

Trying n = 3:

μ = √[3(3 + 2)] = √15 ≈ 3.87 B.M.

So number of unpaired electrons = 3

Now check options:

Ti²⁺ → d² → 2 unpaired ❌ V²⁺ → d³ → 3 unpaired ✅ Cr²⁺ → d⁴ → 4 unpaired ❌ Mn²⁺ → d⁵ → 5 unpaired ❌

Hence, correct metal ion is V²⁺.