Concept: The question tests on basics of Circles
Since O is the Centre, the length of minor arc DE = Angle DOE
Angle BCO = 180 - 140 = 40 (Sum of angles on a straight line = 180
Also angle OBC = 90 as the radius and the tangent are always at 90.
Angle DOE = 90 + 40 = 130 (Exterior angle = Sum of opposite interior angles)