The correct answer is A. constant.
A constant is a value that does not change. In the regression equation $Y = $6000 + (per unit cost x number of units), the value $6000 is a constant. It does not change regardless of the value of the other variables in the equation.
A variable is a value that can change. In the regression equation $Y = $6000 + (per unit cost x number of units), the values of the per unit cost and the number of units are variables. They can change depending on the situation.
An expression is a combination of numbers, variables, and operators. In the regression equation $Y = $6000 + (per unit cost x number of units), the expression $(per unit cost x number of units)$ is an expression. It is a combination of the variable per unit cost and the variable number of units.
A base and exponent are two parts of an exponential expression. In the regression equation $Y = $6000 + (per unit cost x number of units), there is no exponential expression. Therefore, there is no base and exponent.