Final answer:
The object whose emission line is observed at 693 nm when its rest wavelength is 700 nm is moving towards us, as demonstrated by the Doppler effect which shortens the observed wavelength.
Step-by-step explanation:
In physics, the observed shift in wavelength of a spectral line can provide information about the motion of an object. When the observed wavelength of a line is longer than the rest wavelength, it indicates that the object is moving away from the observer. So, in this case, if the rest wavelength of the line is 700 nm but we observe it at 693 nm, it means that the object is moving away from us.