Betelgeuse and Barnard's Star Are Both in the Red Portion

Betelgeuse and Barnard's star are both in the red portion of the H-R diagram as shown in Figure 17-15. What does this tell you about their relative sizes?

A) All red stars are more or less the same size.
B) Barnard's star is still on the main sequence. Betelgeuse has evolved off the main sequence, losing much of its mass in the process. Thus it is smaller than Barnard's star.
C) The figure shows that both stars have the same surface temperature. Since they have the same temperature and the same color, they have the same size.
D) Betelgeuse has expanded to become a giant and is thus much larger than Barnard's star.