So, we need to find the model that does not = 2/6.
2/6 = 2/3 and 4/12 [ There are more, bu those are the most common. ]
a = 1/3
b = 1/3
c = 4/12
d = 3/6
The correct answer is D because 3/6 does not equal 2/6.
Glad I could help, and good luck!
AnonymousGiantsFan