Geometry

Question Analysis:
This question requires determining perpendicular sides using slopes.
The main focus is on identifying perpendicularity.

Key Concept Explanation:
Perpendicular lines have slopes that are negative reciprocals.
Horizontal and vertical lines are always perpendicular.

Step-by-Step Solution:
Slope of AB = (1 − 1)/(4 − 1) = 0 (horizontal)
Slope of BC = (5 − 1)/(4 − 4) → undefined (vertical)
Slope of AD = (4 − 1)/(1 − 1) → undefined (vertical)
Slope of CD = (4 − 5)/(1 − 4) → 1/3
Horizontal and vertical lines are perpendicular.

Option Analysis: