Mark, a Civil Engineer, Entered into a Contract with David

Mark, a civil engineer, entered into a contract with David. As per the contract, Mark agreed to design and build a house for David for a specified fee. Mark provided David with an estimation of the total cost and the contract was mutually agreed upon. However, during construction, when Mark increased the price stating a miscalculation on his part, David refused to pay the amount. This scenario is an example of a _____ mistake.

A) mutual
B) collective
C) collateral
D) unilateral