What is the definition of actual cost-per-click (CPC)?

What is the definition of actual cost-per-click (CPC)?

• A) The average CPC the advertiser needs to pay in order to achieve top position.
• B) The least possible CPC the advertiser needs to pay to maintain an achieved position.
• C) The CPC according to a price list, which is then updated daily.
• D) The CPC an advertiser was charged minus credits for overshot daily budget.