Which of the following triangle measurements represents a triangle similar to one with measurements 21, 13, and 19 inches?

Explain or show work.
a) 11.5 inches, 7.5 inches, 10.5 inches
b) 9.5 inches, 5.5 inches, 8.5 inches
c) 10.5 inches, 6.5 inches, 9.5 inches
d) 12.5 inches, 8.5 inches, 11.5 inches