Assuming that the random process \(U(t)\) is wide-sense stationary, with mean \(\bar{u}\) and variance \(\sigma^{2}\), which of the following functions represent possible structure functions for \(U(t)\) ? Explain each answer.

(a) \(D_{U}(\tau)=2 \sigma^{2}[1-\exp (-\alpha|\tau|)]\),

(b) \(D_{U}(\tau)=2 \sigma^{2}[1-\alpha|\tau| \cos \alpha|\tau|]\),

(c) \(D_{U}(\tau)=2 \sigma^{2}[1-\sin \alpha \tau]\),

(d) \(D_{U}(\tau)=2 \sigma^{2}[1-\cos \alpha \tau]\),

(e) \(D_{U}(\tau)=2 \sigma^{2}[1-\operatorname{rect} \alpha \tau]\).