Answer:
B. a super-giant in spectral class K
Step-by-step explanation:
The classification of stars are done on various characteristics like surface temperature, luminosity, spectral class etc. HR diagram is a plot which shows the relation between these characteristics of stars. One can also predict the life cycle of stars. When you plot the stars basis these characteristics on HR Diagram you can easily see that the stars with temperature lower than 4000K and brightness several thousand of that of the Sun will be a supergiant star.