An astronomer discovers an object at a distance of 28 Mpc from the Earth. (Recall, M-prefix meaning Mega, i. e. times 106) At this distance, what is the object most likely to be?

The object is most likely to be a (distant) galaxy

Explanation:

Distance of galaxies are often calculated based on the unit of light year.

The abbreviation, mpc which translates to mega parsec

A mega parsec is equivalent to 3.3 light years.

So, if the astronomer sighted the object from a distance of 28mpc which is equivalent to 28 * 3.3 light-years (92.4 light years); the object is most likely to be a galaxy; a distant galaxy