Cost Plus Percentage of Cost contracts are more desirable for the?

A cost-plus contract, more accurately termed a Cost Reimbursement Contract, is a contract framed in such a way that when the contractor finishes the agreed-upon work, they receive compensation equal to their expenses plus a profit. Cost reimbursement contracts contrast with fixed-price contract, in which the contractor is paid a negotiated amount regardless of incurred expenses

Cost Plus contracts (also referred to as 2 limbs of payment eg limb 1 cost and limb 2 fixed overhead and profit) in standard form suffer from having no competitive drivers. An example is a "day works" arrangement where there is a high degree of uncertainty in what has to be done but a certainty around how it is to be done eg equipment for repairs to a burst water main located in the middle of a busy interestion. This would be almost impossible to price as there its an emergency and highly uncertain.  This arrangement is convenient and suits both parties in the short term. However there are innovative procurement models available in the market place designed to address the issue of competitive forces for programs of work that encourage and reward efficiency and deliver a better outcome for all parties involved. I have developed one such model called a Portfolio Management Partnership, that reduced costs, expedited work to market and improved value for money outcomes for everyone. For this to work there needs to be the appropriate legal instrument combined with a specifically designed administrative and commercial arrangements that complement the legal instrument applied to a sufficiently large enough program of work that will allow principles to be realised. 

Cost plus with the right competitive drivers will benefit all parties. 

Thanks for the invitation 

B. Both seller and contractor if the Contract Costing is transparent 

B. Both1) seller and2) contractor

Option "B". For seller and contractor both.

I would go for option B Both1) seller and2) contractor

for both the seller and the contractor is good as they are free of any risks and transfer it to the buyers and owners

I would select B, Both of them as a shared cost.

Option B is the correct answer.

Seller Of course, I am working in a Tunnel Project, that contract is basically One Sided.

Depends on the situation:

Generally speaking - B