Measurements Show a Certain Star Has a Very High Luminosity

Multiple Choice

Measurements show a certain star has a very high luminosity (100,000 x the Sun's) while its temperature is quite cool (3500^o K) . How can this be?

A) it must be a main sequence star
B) it must be quite small in size
C) it must be quite large in size
D) it must be brown dwarf and not a regular star
E) this must be an error in observations; no such star can exist