(c) (3, 4)
In rectangle ABCD, with B(0,0), C(3,0), D(0,4): since AB ∥ DC and AD ∥ BC, vertex A must have x-coordinate of C (i.e., 3) and y-coordinate of D (i.e., 4). So A = (3, 4).
Source: Chapter 7, Coordinate Geometry
In rectangle ABCD (vertices in order), BC lies along the x-axis and BD along the y-axis. A is diagonally opposite to B, so it shares its x-coordinate with C (x = 3) and its y-coordinate with D (y = 4), giving A(3, 4). You can verify: AB = CD = 4 units (vertical), BC = AD = 3 units (horizontal) — consistent with a rectangle.