Hi Pawan Ramnani,
This Geometry question tests you on your knowledge of some standard circle and line rules:
1) A circle is 360 degrees
2) Lines add up to 180 degrees
3) Opposite angles are equal.
In this circle, we see a pair of criss-crossing lines. The two angles next to the 120 are both 60 (since lines add up to 180 degrees). The angle across from the 120 is also 120 (which totals 360 degrees. Thus, the two "Y" sections total 60+60 = 120 degrees.
120 degrees/360 degrees = 1/3
Final Answer: C
GMAT assassins aren't born, they're made,
Rich