Which equation does the graph below represent?
y = fraction 1 over 3 + x
y = fraction 1 over 3x
y = 3 + x
y = 3x

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!

See where the line crosses the y axis and then find the slope

Understanding what? lol