Hello Markus,
My contribution...I would say item a. Cost Plus Fixed Fee (CPFF) presents the most risk for the buyer. There is less incentive for the seller to keep control of costs than with a fixed-price contract and the buyer would need to provide resources to oversee the costs to make sure they are reasonable. In the case of the Cost Plus Award Fee (CPAF), a board is generally assigned to review the quality of the work done and bases the award on those findings. According to PMBOK 5th edition bottom of page 344, “In many cases, use of a cost-plus contract may transfer the cost risk to the buyer, while a fixed- price contract may transfer risk to the seller.” Hope this helps.