Exercise 9.18 An harmonic oscillator and Nyquist plots

(a)

(b)

An answer:
(c)

(c)

An answer:
no, $\omega =0$ may result but that is not regarded an harmonic oscillation.

(d)

An answer:
yes, at $\omega =\frac{\mathit{tan}(60^{\circ }}{\mathit{RC}}$