Assertion: [Ni(CN)₄]^2- is diamagnetic complex.
Reason: [Ni(CN)₄]^2- involves dsp² hybridisation and there is no unpaired electron.

Assertion and reason both are correct statements and reason is correct explanation for assertion.

In [Ni(CN)₄]^2- 

x + 4(-1) = -2

x - 4 = -2

x = -2 + 4 = 2

₂₈Ni : [Ar]4s²3d⁸

Ni²⁺ : [Ar]3d⁸ 

[Ni(CN)₄]^2- is diamagnetic complex. It involves dsp² hybridisation and there is no unpaired electron.