Answer:
Option C is correct option.
Step-by-step explanation:
We need to find k constant of proportionality
In the graph below let us take the values of x and y
x=150 , y=50
x=300, y=100
x=450, y=150
Now finding k using the formula: 
Putting values of x and y to find k
For x=150 , y=50 k = 50/150, k =1/3
For x=300, y=100 , k=100/300 , k=1/3
For x=450, y=150, k= 150/450 , k=1/3
Since for values of x and y, k = 1/3 is constant so, the graph shows two quantities that vary directly.
So, Option C is correct option.