Geometry

Question Analysis:
This question tests knowledge of exterior angles in an isosceles triangle, requiring understanding of both triangle properties and the Exterior Angle Theorem.

Key Concept Explanation:
In an isosceles triangle, angles opposite equal sides are equal.
The Exterior Angle Theorem states that an exterior angle equals the sum of the two non-adjacent interior angles.

Step-by-Step Solution:
Since DE = DF, angles E and F are equal. Let each measure x.
Sum of angles in triangle DEF: 40° + x + x = 180° → 2x = 140° → x = 70°.
Exterior angle at E = angle D + angle F = 40° + 70° = 110°.