Identify neutral complex from following.

Identifying a neutral complex requires calculating the total charge of each coordination entity. The overall charge is the sum of the central metal's oxidation state and the charges from all ligands.

For tetracyanonickelate(II) ion, the nickel center is Ni²⁺ and four CN^– ligands contribute –4, yielding a –2 charge: $+2 + 4 \times (-1) = -2$, an anion.

Sodium hexafluoroaluminate(III) contains the complex ion [AlF₆]^3–, as aluminum is +3 and six F^– ligands contribute –6: $+3 + 6 \times (-1) = -3$, an anion.

Triamminetrinitrocobalt(III) has cobalt at +3, three neutral NH₃ ligands, and three NO₂^– ligands contributing –3: $+3 + 3 \times 0 + 3 \times (-1) = 0$, a neutral complex.

Tetraamminecopper(II) ion features Cu²⁺ and four neutral NH₃ ligands: $+2 + 4 \times 0 = +2$, a cation.

$\boxed{C}$ is the neutral complex.