First of all, recognize that the powers of x are even, so the "x" in the given form will actually be x². The given form has a coefficient of 1 for the squared term (here, x⁴), so we must divide by the coefficient of that term.
... x⁴ - (5/2)x² -12/2 = 0
Putting the constant term on the other side of the equal sign (by adding its opposite), we get
... x⁴ -2.5x² = 6
To complete the square on the left, we add the square of half the coefficient of the "x" term (here, x² term).
... x⁴ -2.5x² +1.25² = 6 + 1.25²
Now we can rewrite the left side to the desired form and add the terms on the right.
... (x² -1.25)² = 7.5625 . . . . . corresponds to D)