B. The cost of orange juice is $3 per jug
A unit rate, in terms of mathematics, is a rate per unit.
Examples of unit rates are:
The car travels 25 km per hour
The cost of bags of apples is $6.00 per bag of apples.
The unit rate in these choices is Choice B:
The cost of orange juice is $3 per jug
The other choices are rates, not unit rates.