The correct answer is (b) 4000 Hz
The best explanation: Given x(t) = 1 + cos 2000πt + sin 4000πt
Highest frequency component in 1 is zero
Highest frequency component in cos2000πt is ωm1 = 2000π
Highest frequency component in sin4000πt is ωm2 = 4000π
So the maximum frequency component in x(t) is ωm = 4000π [highest of 0, 2000π, 4000π]
∴ 2πfm = 4000π
2fm = 4000
Nyquist rate, Fs = 2fm = 4000 Hz.