Final answer:
The correct equation representing the difference between the lengths of two blocks being equal is x - 20 = 0, signifying that their lengths are the same.
Step-by-step explanation:
To create an equation that relates the difference between the length of the purple block, x, and the length of the green block, which is 20, we simply express this difference as an equation. Because we are talking about the difference, the correct form of the equation would be x minus 20. Thus, the correct answer would be x - 20 = 0 (Option A). This signifies that the length of the purple block is exactly the same as the green block, making their lengths equal when the difference is zero.