If x - 1/x = 4 , then ( x + 1/x ) is equal to?

Answer: Option B

$$\begin{array}{rl}& x-\frac{1}{x}=4\\ & \left(\text{on squaring }\right)\\ & \Rightarrow x^2+\frac{1}{x^2}-2=16\\ & \Rightarrow x^2+\frac{1}{x^2}=18\\ & \Rightarrow x^2+\frac{1}{x^2}+2-2=18\\ & \Rightarrow x^2+\frac{1}{x^2}+2=20\\ & \Rightarrow {\left(x+\frac{1}{x}\right)}^2=20\\ & \Rightarrow x+\frac{1}{x}=\sqrt{20}\\ & \Rightarrow x+\frac{1}{x}=\sqrt{4\times 5}\\ & \Rightarrow x+\frac{1}{x}=2\sqrt{5}\end{array}$$