Correct Option: A

We know that spectral density of is the Fourier transform of autocorrelation function.
Let the autocorrelation function R_xx.
(A) R_xx(τ) = cos ω₀τ

Rxx(τ) =	e– jω0T + e– jω0T
	2

or S_xx(ω) = ∫^∞_{– ∞} R_xx e^{– jωτ}dτ

or Sxx(ω) = ∫∞– ∞		e– jω0τ + e– jω0τ		e– jωτdτ
		2

or S_xx(ω) = π[δ(ω – ω₀) + δ(ω + ω₀)]
(by using frequency shifting property)
There, no need to solve other alternative. This is the correct answer.