For the given equation:
m = 3b
The constant of proportionality is 3, so the correct option is A.
How to determine the constant of proportionality?
After a small search on the internet, I've found that this question asks for the constant of proportionality.
Remember that a proportional relation is of the form:
y = k*x
Where k is the constant of proportionality, and x and y are the variables.
Here the relation is:
m = 3*b
Where m and b are the variables, then the remaining coefficient is the constant of proportionality, which is 3.
In this way, we can see that the correct option is A.