Let's do the actual work to be able to answer this question.
Subtract x^6 - 2ax + c from x^2 - ax.
You may find it easier to write these two expressions in a column:
x^2 - ax
-( x^6 - 2ax + c)
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-x^6 + x^2 + ax - c are the four terms that result from this subtraction. -x^6 is the only one listed among the possible answer choices.