Remark
There are two conditions that must be met to answer this question.
The central angle angle of an arc is double any angle touching the circumference of a circle on the same side as the central angle. That means that a is 50 degrees, which is not your question.
The second condition is that if condition 1 is met and there are other angles that also touch the circumference, then they are equal to 1/2 the central angle if and only if they have the same end points as the central angle. In this case b does share the same end points as the central angle.
Therefore the answer is b = 25 degrees
Comment
Follow the diagram below. C and D are the end points of the central angle COD. So COD is double the size of the given angle . b shares the same end points as a and the given 25 degree angle. From condition 2 above. <b = the given angle of 25 degrees. I'm assuming the answer is listed and your picture has been cut off so that the answer was not displayed. But your question has it. The answer is D.