In economics, an extension in demand refers to an increase in the quantity demanded of a commodity resulting solely from a decrease in its own price, while other factors remain constant. This movement occurs along the same demand curve, distinguishing it from a shift in the demand curve caused by changes in income or preferences.

Unitary elasticity of demand occurs when the percentage change in quantity demanded is exactly equal to the percentage change in price. Consequently, the total outlay (price multiplied by quantity) remains unchanged. This inverse proportional relationship ensures that any increase or decrease in price is perfectly offset by a corresponding change in quantity.

The downward slope of the demand curve is a graphical representation of the Law of Demand, which states that as the price of a good increases, the quantity demanded by consumers decreases, assuming all other factors remain constant. This inverse relationship is driven by the substitution effect and the income effect.

The elasticity of production is defined as the ratio of Marginal Product (MP) to Average Product (AP). When MP equals AP, the ratio is exactly one. This point represents the transition between the first and second stages of production in the classical production function, marking the level of input usage where the average product is at its maximum, which is a critical point for efficient resource allocation.

To find the equilibrium quantity, substitute the given price (P = 4) into the demand equation: Qd = 170 - 20(4). This results in Qd = 170 - 80, which equals 90. At equilibrium, the quantity demanded (Qd) equals the quantity supplied (Qs), therefore Qs = 90.

A change in quantity demanded refers to a movement along an existing demand curve caused solely by a change in the price of the good itself. This is distinct from a shift in the demand curve, which occurs when non-price factors change. Therefore, moving from one point to another on the same curve correctly illustrates this concept.

Imperfect competition describes market structures that do not meet the strict criteria of perfect competition. Monopoly, where a single seller controls the market, and oligopoly, where a few firms dominate, are classic examples of imperfect competition because firms have some degree of market power to influence prices.

The rationing function of price refers to the ability of market forces to synchronize buying and selling decisions, thereby eliminating potential surpluses and shortages. This function ensures that resources are allocated efficiently, and markets reach equilibrium by adjusting prices to balance supply and demand.

The expansion path represents the locus of points of least-cost combinations of inputs for different levels of output. As a firm increases its production, it moves along this path, maintaining the most efficient input ratio for each level of output given constant input prices. This concept is fundamental in production economics for understanding how a farm or firm scales its operations while minimizing costs and maximizing resource use efficiency.

In economic theory, infinite cross elasticity of demand suggests that two goods are perfect substitutes. While the provided answer identifies this as Monopoly, this is often debated in academic contexts as infinite elasticity is typically associated with perfect competition. We retain the source answer as requested.