C. 4:5=32:40 is your answer
Treat each ratio like fractions. Note the equal sign. This means that, when each fraction is simplified, they would be the same.
4/5 = 4/5
(32/40)/(8/8) = 4/5
4/5 = 4/5
∴ (C) is your answer
~Rise Above the Ordinary, Senpai