The correct option is (a) A/√7
Explanation: Let x = Asin(wt).
U = kA^2sin^2(wt) K = 1/2mA^2w^2cos^2(wt). Given: U = 1/3K. kA^2sin^2(wt) = (1/3)*(1/2)mA^2w^2cos^2(wt),
and k = mw^2.
∴ tan^2(wt) = 1/6. Or tan(wt) = 1/√6.
∴ sin(wt) = 1/√7. x = Asin(wt) = A/√7.