Accurate choice is (c) Vo= { Vx×Vy× [cosθ-(cosθ×cos2ωt)+(sinθ×sin2ωt)]}/(2×Vref)

Simplify with an easy explanation: The output of frequency doubler, Vo= (Vx×Vy)/Vref = [(Vsinωt +Vsin(ωt+θ)]/ Vref

= {Vx×Vy×[(sin^2ωt×cosθ)+(sinθ×sinωt×cosωt)]}/ Vref

=> Vo= { Vx×Vy× [cosθ-(cosθ×cos2ωt)+(sinθ×sin2ωt)]}/(2×Vref).