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