The correct option is D (a) and (b) above.
A diagonal is a line segment connecting two non-consecutive vertices of a polygon.
Here, A and C are non-consecutive vertices of the polygon. Hence the line segment joining A and C will be a diagonal.
Similarly, ¯¯¯¯¯¯¯¯¯BD is also a diagonal,
Thus, ¯¯¯¯¯¯¯¯AC and ¯¯¯¯¯¯¯¯¯BD are the diagonals.