The equations that represent circles that have a diameter of 12 units and a center that lies on the y-axis are:
1. (x - 0)² + (y - 6)² = 36 (center at (0, 6))
2. (x - 0)² + (y + 6)² = 36 (center at (0, -6))
Therefore, options (1) and (2), which involve a radius of 6 units and a center at either (0,6) or (0,-6), respectively, satisfy the requirements mentioned for the circle.