NPPE materials define a unit-price contract as one in which payment is based on measured quantities of work performed at predetermined unit rates. This contract type is commonly used when the total quantity of work cannot be precisely determined at the outset.

Option B correctly reflects this definition. Cost-plus contracts reimburse actual costs plus a fee, fixed-price contracts pay a single lump sum, and design-build contracts combine design and construction responsibilities rather than defining a payment structure based on units.

Therefore, option B is the correct answer.