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I got choice C
Working with a)
There are two possiblities
1. The quadilateral is a rectangle (if two interior angles are opposite to each other)
2. The quadilateral has two parallel sides (if the angles are adjacent to each other)

The condition is therefore insufficient

Working with b)
Nothing can be assessed.

When both are used
The first possiblity of condition a is ruled out.
In the second possiblity, since the lines are parallel, the sum of interior angles should be 180
therefore
angle ABC + Angle BCD=180
ie 2x+x=180
Therfore x=60