The answer is called a linear equation :))
1=5-4 because you have to substitute x and y (x,y) or (5,1)
The ratio is always 1:1:√2
For this case we have that by definition, a direct variation is given by:

Where:
k: It is the constant of proportionality of the variables.
On the other hand, we have that the inverse variation is given by:

Where:
k: It is the constant of proportionality of the variables.
In this way, the correct option is: 
ANswer:
Option A