Which proportion must be true?

Responses
A
10
5 = 11
b 10 5 = 11 b
B
10
5 = b
11 10 5 = b 11
C
8.5
5 = b
8.5 8.5 5 = b 8.5
D
10
b = 11
5

10/5 = 11/b