An HR diagram is a plot of what two stellar properties

A absolute magnitude and luminosity

B age and mass

C luminosity and temperature

D mass and absolute magnitude

E temperature and spectral type

i would say C, though normally an Hertzsprung-Russell (H-R) Diagram, shows COLOR and luminosity, but color is closely related to its temperature.

You may as well just say B which will give you the mark you deserve (or if you weren't so busy cheating you could have simply read the axes on one of the plots).