Final answer:
Statement 6, based on the definition of a bisector, justifies that diagonals AC and BD bisect each other at E in a parallelogram.
Step-by-step explanation:
The question involves proving that in a parallelogram, the diagonals bisect each other. The correct statement that justifies that AC and BD bisect each other at E is Statement 6, which is derived from the definition of a bisector. This definition states that a bisector divides a segment into two equal parts, hence AE = CE and BE = DE. Since AE and CE are equal, and BE and DE are equal, it follows that AC and BD bisect each other at point E, in accordance with the properties of a parallelogram.