I assume you mean what is it reflected across? If so, then actually, none of those answers are right in general. If f is even, then f(-x) is symmetric across the y-axis; if f is odd, then f(-x) is symmetric across the origin; and if f is neither even nor odd, it could be symmetric across anything or nothing, we would need more information to decide.
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C. line x=y
I assume you mean what is it reflected across? If so, then actually, none of those answers are right in general. If f is even, then f(-x) is symmetric across the y-axis; if f is odd, then f(-x) is symmetric across the origin; and if f is neither even nor odd, it could be symmetric across anything or nothing, we would need more information to decide.