Here is your answer
Both
a) /_D
c) /_ADB
are correct
REASON:
/_ADB represents an angle between AD and BD which is marked in the question.
/_D represents the point where the two lines or rays have a common point, i.e.D
[However writing a single letter to represent an angle is not completely correct. But when there is only one angle represented through the point then writing a single letter is correct
i.e. /_D is correct ]
HOPE IT IS USEFUL