Selection C is appropriate.
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Whenever you solve for something, consider what operations are done to that something. Undo them in reverse order.
Here, "a" is
• multiplied by 2
• has -b^2 added
The last operation is adding -b^2, so we undo that first. We undo it by adding its opposite, b^2.
.. x +b^2 = 2a -b^2 +b^2
.. x +b^2 = 2a . . . . . . . . . . . the point is to cancel the -b^2 term on the right.
The operation before that is multiplying "a" by "2". We undo that by multiplying by the reciprocal of 2, which is 1/2.
.. (1/2)*(x +b^2) = (1/2)*2a
.. (x +b^2)/2 = a . . . . . . . . . the point is to cancel the multiplication by 2 on the right.
You will note this matches answer selection C.