Which of the following is NOT a type of construction contract?

The correct answer highlights that "flat rate" is not a recognized type of construction contract. In construction, contracts are typically categorized into specific types based on their pricing structures and payment terms.

Lump sum contracts involve a single fixed price for all work performed, which provides clarity and simplicity for budgeting. Cost-plus contracts allow for reimbursement of actual costs plus a fee for overhead and profit, making them more flexible for projects with uncertain scopes. Unit price contracts are based on a fixed price per unit of work completed, which can be beneficial for projects that involve variable quantities.

"Flat rate," while it may describe certain pricing agreements in other contexts, is not a formal term used in construction contract nomenclature. Thus, it does not fit within the established types of contracts recognized in the industry, making it the correct choice for this question.