Geometry

Question Analysis:
Tests application of midpoint congruence in coordinate geometry.

Key Concept Explanation:
In parallelograms, the midpoints of both diagonals coincide (are identical).

Step-by-Step Solution:
Calculate midpoint of AC:
((1 + 5)/2, (1 + 5)/2) = (3, 3)
Let D = (x, y). Midpoint of BD must be (3, 3):
((4 + x)/2, (2 + y)/2) = (3, 3)
Solve:
(4+x)/2 = 3 → x = 2
(2+y)/2 = 3 → y = 4

Option Analysis:
A: Correct (matches calculation)
B: Incorrect (would be midpoint if D...