How Can Astronomers Determine the Size of an Emission Region

How can astronomers determine the size of an emission region in a very distant and unresolvable source?

A) by measuring the object's mass and using a reasonable value for the average density for matter to calculate its volume and hence its diameter
B) by using radio interferometry because this technique can resolve far greater detail than optical imaging
C) by measuring brightness variability because an object cannot vary more rapidly than the time taken for light to cross the source
D) by measuring the redshift of its spectrum because the redshift will depend on the source size