Which of the following contract types is best used if the scope is well defined but not the quantities of material?

The best contract type to use when the scope of work is well defined but the quantities of materials are uncertain is the unit price contract. This type of contract allows for payment to be based on the actual quantities of work completed rather than an overall fixed price.

In scenarios where the project scope is clear, but the quantities may fluctuate due to site conditions, design changes, or other uncertainties, a unit price contract provides flexibility. The contractor submits a price per unit of work, and payments are made as work is executed, based on measured quantities. This mitigates risks associated with estimating the total price upfront when quantities are not strictly known.

In contrast, lump sum contracts require a fixed price for all work based on an estimated scope and quantities, making them less appropriate when actual quantities can't be determined. Cost plus contracts and cost plus with Guaranteed Maximum Price (GMP) contracts do provide flexibility and can accommodate variations in scope and cost, but they typically involve more administrative oversight and potential cost unpredictability, which may not be necessary when the work scope is clear but quantities are uncertain. Therefore, the unit price contract stands out as the optimal choice for this particular situation.