Answer:
A. x-axis
Explanation:
The answer is the x-axis because the trangle is an exact reflection flipped across the x-axis. We also know this was flipped across the x-axis because the x values do not change, but the y values are the exact opposite. For example, point M has coordinates (1,-1) while point M' has coordinates (1,1). This is the same for points K, K', J, and J'.
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