According to the PMBOK® Guide, specifically the Project Procurement Management knowledge area, contracts are generally categorized into three broad types. However, when discussing the most fundamental and common " pillars " of contracting, the industry focuses on how risk is shared between the buyer and the seller.

Fixed-Price Contracts (FP): This category involves setting a fixed total price for a defined product, service, or result. It is used when the requirements are well-defined and unlikely to change significantly. In this model, the seller carries the highest risk, as they are responsible for any cost overruns.

Cost-Reimbursable Contracts (CR): This category involves payments to the seller for all legitimate actual costs incurred for completed work, plus a fee representing seller profit. It is used when the scope of work is not well-defined or involves high risk/uncertainty. In this model, the buyer carries the highest risk, as the final total cost is unknown until the project is complete.

Time and Material Contracts (TandM): While very common, TandM is often considered a " hybrid " type that contains elements of both fixed-price and cost-reimbursable contracts. It is frequently used for smaller engagements, staff augmentation, or when a quick start is needed, but in terms of primary project procurement frameworks, the binary distinction usually falls between Fixed Price and Cost-Reimbursable.

Choice A, C, and D: These choices include specific sub-types (like CPAF) or focus on the hybrid model (TandM). While these are used, they do not represent the two primary categories that define the spectrum of procurement risk as broadly as Choice B.

By selecting the appropriate contract type from these two primary categories, the project manager aligns the procurement strategy with the project ' s risk profile and the clarity of the scope.