If\, loga/b + logb/a = log(a + b), then:

Answer: Option A

$$\begin{array}{rl}& \log \frac{a}{b}+\log \frac{b}{a}=\log \left(a+b\right)\\ & \Rightarrow \log \left(a+b\right)=\log \left(\frac{a}{b}\times \frac{b}{a}\right)=\log 1\\ & So,a+b=1\end{array}$$