Which equation is quadratic in form?

An equation is quadratic in form when its powers can be rewritten as a quadratic in one expression, such as letting u = x⁴ and getting au² + bu + c = 0. In A, x⁸ = (x⁴)², so the equation becomes 10u² + 7u + 1 = 0. The other choices do not have powers that form a quadratic in a single power of x.

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