Since the options have not been given, I am giving an assumed answer.
A rate is defined as the ratio in which two different kinds of numbers or units are compared. A unit rate allows comparison of the to its units of measurement. Unit rates are expressed as a single quantity or as a quantity of 1. Examples - 5 miles per hour, 2 dollars per pound, 4 feet per second, etc.
So, the best way to calculate the unit rate for a proportional relationship from a graph will be to first check the graph of the relationship. The next step will be to find the y-value of that particular point which corresponds to the point where x = 1. That particular point value is calculated as the unit rate.