= Solution
Reading the time axis, the rapid oscillation has period about $2\ \mathrm s$, while successive envelope maxima are about $10\ \mathrm s$ apart. The envelope stays nonzero and has a slowly sinusoidal modulation about a positive mean. The corresponding amplitude-modulation model is $y=[A+B\cos(2\pi\,0.1t)]\cos(2\pi\,0.5t)$, with $A>|B|$. Product-to-sum gives
$$
y=A\cos(2\pi\,0.5t)+\frac B2\cos(2\pi\,0.6t)+\frac B2\cos(2\pi\,0.4t).
$$
Thus the intended positive-frequency components are \b[$0.4$, $0.5$ and $0.6\ \mathrm{Hz}$]. The envelope <frequency> $0.1\ \mathrm{Hz}$ is not itself a separate additive displacement component. These values use the illustrated sinusoidal-modulation interpretation; a finite qualitative trace alone does not uniquely specify every possible spectral component.
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