So think this tricky question through really carefully.....
If the propeller rpm is fixed and the airspeed increase, the angle of attack of the blade:
That's very good point. The wording of the question is not precise enough to discriminate between the given answer choices. Stated another way, either answer A or answer B could be correct depending upon how the RPM is held constant.
I think the question would be more valid if the question were worded like this:
With a fixed pitch propeller and fixed throttle position, an increase in airspeed would result in the following effect upon blade angle of attack:
Perhaps a better wording?
Another example of how much thought is required in order to write good test questions!