Angle C is the right angle, since it is an inscribed angle 
leaning on the diameter of the circle.


Hence, angle B in triangle ABC is 90 - 30 = 60 degrees.


Since the lines BC and DE are parallel, we conclude that the angle DEB is 60 degrees.

Hence, the angle ADE is 180-60 = 120 degrees.


Triangle ADE is an isosceles triangle;  therefore, angle D is half of 180-120 = 60 degrees.


ANSWER.  Angle D is 60/2 = 30 degrees.