Step 1 :
The equation of the square formed by the lines are $x=0, y=0, x=1 \: and \: y=1$ respectively.
Hence the vertices of the square are $0(0,0), A(1,0), B (1,1), C (0,1)$ respectively
Hence the equation of the diagonals 0B is
$ \large\frac{y-y_1}{y_2-y_1}$$= \large\frac{x-x_1}{x_2-x_1}$
(i.e) $ \large\frac{y-0}{1-0}$$=\large\frac{x-0}{1-0}$
$ \Rightarrow y=x$--------(1)
equation of the diagonal AC is
$ \large\frac{y-0}{1-0}$$ = \large\frac{x-1}{0-1}$
$ \Rightarrow -y=x-1$
$ \Rightarrow x+y=1$
Hence 'A' is the correct answer.