The correct option is C 240∘
Consider that arc AXB is the minor arc and arc AYB is the corresponding major arc. It is known that, measure of major arc = 360∘ − measure of the corresponding minor arc.
We have, m(arc AXB) = 120∘ .
So, m(arc AYB) = 360∘ − m(arc AXB) = 360∘−120∘=240∘