Answer:
b.location in the sky (If the question is saying:The H-R diagram is useful in
studying all of these about a star except for)
Step-by-step explanation:
The theoretical HR diagram plots the temperature of the star vs their luminosity (brightness) and the observational HR diagram plots color of the stars vs the absolute magnitude. Depending on mass, stars will also go through specific evolutionary stages that can be seen through the location of the star on the HR diagram. However, the HR diagram will not have data on the location of the star in the sky.