Market value is best defined as:

Market value is best understood as the amount a willing seller would accept and a willing buyer would pay. This definition reflects the fundamental principles of supply and demand in real estate transactions. It emphasizes that both parties are engaged in a fair negotiation without pressure or undue influence, thereby arriving at a price that both find acceptable.

This concept is rooted in the concept of "willingness", indicating that both the seller and buyer are motivated by their individual circumstances and interests. The market value is determined in an open and competitive marketplace, where potential buyers have the opportunity to assess the property and make informed offers.

In contrast to market value, options like the price at an auction, an appraiser's assessment value, or the average price of comparable properties lack the same emphasis on the voluntary nature of the transaction. Auction prices can fluctuate greatly depending on the bidding environment and might not represent true market conditions. Assessment values may not reflect current market trends as they are often based on different criteria and methodologies. Average prices of similar properties can provide insight but do not account for specific conditions surrounding a particular sale. Thus, the essence of market value lies in the mutual agreement reached between a seller and buyer, free from constraints, which aligns perfectly with the chosen answer.