= Solution
{c}
The <small-angle approximation> $\sin\theta=\theta+O(\theta^3)$ reduces <6/b/ii> to $\ddot\theta+\omega^2\theta=0$, where
$$
\boxed{\omega=\sqrt{\frac g\ell\,
\frac{\frac12+\mu\delta}{\frac13+\mu(\delta^2+\alpha^2/2)}}.}
$$
The coefficient is positive, so the solution is <simple harmonic motion>, $\theta(t)=C\cos\omega t+D\sin\omega t$. Here $\omega$ is the <angular frequency>; the ordinary <frequency> and <period> are
$$
\boxed{f=\frac\omega{2\pi},\qquad T=\frac{2\pi}{\omega}.}
$$
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