Answer: The Third Circle
Step-by-step explanation: If you simplify all the circles into fraction (like what you did) you will see that:
the first circle equals 1/3
the second circle equals 2/6 which is simplified to 1/3 also
the third circle equals 2/5 which can't be simplified any further
The fourth Circle equals 4/12 which is simplified to 1/3 too
So the third circle doesn't belong because it doesn't equal 1/3