Concept:Use the tangent addition formula after simplifying the inverse trigonometric angles.Explanation:Let θ=cos−1(21) and ϕ=tan−1(21).Since cosθ=21, we get θ=4π, so tanθ=1.Also, tanϕ=21.Now, the given expression is tan(θ+ϕ).Using the formula tan(θ+ϕ)=1−tanθtanϕtanθ+tanϕ:tan(θ+ϕ)=1−1⋅211+21=2123=3.Answer:3, i.e. Option C.