To solve this problem and find the constant of proportionality for the cost of online streaming, we can analyze the graph provided. The cost is proportional to the number of months, meaning the relationship between the cost (y) and the months (x) follows a linear equation of the form $y = kx$, where $k$ is the constant of proportionality (cost per month).

Looking at the graph, we can identify two points where the line intersects the grid:

1. At $x = 2$ months, the cost $y = 30$ dollars.
2. At $x = 4$ months, the cost $y = 60$ dollars.

Using either of these points, we can calculate the constant $k$:

For the first point:

$k = \frac{y}{x} = \frac{30}{2} = 15$

So, the constant of proportionality is $15$ dollars per month.

Thus, the correct answer is $15 per month.

Would you like any further details or clarification?

Here are some related questions to extend your understanding:

1. How would you calculate the total cost after 10 months based on this constant of proportionality?
2. If the cost per month increased by 5 dollars, what would the new constant of proportionality be?
3. How can we verify the constant of proportionality using a different pair of points on the graph?
4. What would happen to the line on the graph if the streaming service gave a 10% discount per month?
5. Can you write the equation of the line for the total cost in terms of the number of months?

Tip: When dealing with proportional relationships, the constant of proportionality is the rate of change, which is the same as the slope of the line in a graph like this.