The signal $m(t)=cos(\omega_mt)$ is $SSB$ (single side-band) modulated with a carrier $\cos(\omega_ct)$ to get $s(t)$. The signal obtained by passing $s(t)$ through an ideal envelope detector is
- $\cos(\omega_mt)$
- $\sin(\omega_mt)$
- $\cos(\omega_mt)+\sin(\omega_mt)$
- 1