Solution
Expression : $$x^6 - 18x^3 + k$$
Let $$x^3=y$$
= $$y^2-18y+k$$
= $$y^2-2(y)(9)+k$$
For above expression to be a perfect square, $$k=(9)^2=81$$
= $$y^2-2(y)(9)+81$$
= $$(y-9)^2$$
= $$(x^3-9)^2$$, which is a perfect square
=> Ans - (D)
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