The correct answer is: B. learning curve.
A learning curve is a graphical representation of the decreasing amount of time it takes to complete a task as experience is gained. It is often used in manufacturing to predict the time it will take to produce a new product. The learning curve is also used in other industries, such as software development and customer service, to track the improvement of employees over time.
A fixed curve is a line that does not change. It is used to represent a constant value, such as the cost of a product or the number of employees in a company.
A linear curve is a line that has a constant slope. It is used to represent a relationship between two variables that changes at a constant rate.
A mixed curve is a line that has a changing slope. It is used to represent a relationship between two variables that changes at a non-constant rate.
In the context of the question, the function used to measure decline in labour hours per unit as units of production increases is called a learning curve. This is because the learning curve represents the decreasing amount of time it takes to complete a task as experience is gained. As employees gain experience, they become more efficient at completing tasks, which results in a decrease in the number of labour hours per unit.