In Square ABCD all sides are equal =d
In Equilateral Triangle BEC all sides are equal =d
ABCD is a square and BCE is an equilateral triangle.
The angle DCE = 90 + 60 or 150 degrees.
Since BCE is an equilateral triangle, CE = CB = CD, so DEC is an isosceles triangle.
So angle DEC = angle EDC = x degrees
x = (180 - 150)/2 = 30/2 = 15 degrees.
Option A is the Correct Answer.