Here, You need to calculate the slope of the graph by taking any two random points, as follows:
(x₁,y₁) = (0,0) & (x₂,y₂) = (3,1)
Now, we know,
y₂-y₁ = m(x₂-x₁)
1-0 = m(3-0)
m = 1/3
So, Equation would be y = 1/3 x
In short, option B will be your answer.
Hope this helps!