Identity the pair of numbers that does not belong with the other three. explain.

3/5 and 5

In the first pair, the only numbers are 9 and 6. Converting 4 into a fraction leaves us with 4/1 - the only numbers in that pair are 1 and 4. In the final one, the only numbers are 2 and 7.
However, converting 5 gives 5/1. What this means is that the top and bottom numbers are not flipped in this pair, making it the odd one out. 

Hope I helped!

3/5 and 5 is the pair that doesn't belong
The other pairs are made of reciprocal numbers .
9/6*6/9=1
4*1/4=4/1*1/4=1
2/7*7/2=1

but 3/5*5/1=3, not 1.