We are given that \( ABCD \) is a square, so all its angles are 90°. Additionally, \( BCE \) is an equilateral triangle, meaning all its angles are 60°.
We are tasked with finding \( \angle DEC \).
Step 1: The angle \( \angle EBC \) in the equilateral triangle \( BCE \) is 60°.
Step 2: The angle \( \angle DBC \) in the square is 90°.
Since \( \angle DBC = \angle EBC + \angle DEC \), we have:
\[
90° = 60° + \angle DEC
\]
\[
\angle DEC = 30°
\]
Thus, the answer is b. 30°.