Correct option is C

Given:

x + $$\frac{1}{x}$$​ = 2 cos θ

Formula Used:

​$$4 cos^3 θ - 3 cos θ = cos 3θ$$​​

Solution:

​$$x^3 + \frac{1}{x^3} = 8 cos^3 θ - 6 cos θ\\\ \\= 2 [4 cos^3 θ - 3 cos θ]\\\ \\= 2 (cos 3θ)\\\ \\= 2 cos 3θ$$​​