Economists generally use gross domestic product to measure a nation's total output because it (1) is a reliable indicator of the social progress of a nation over time. (2) equals the sales value of all transactions conducted during a period and, thus, can be (3) is unaffected by changes in the prices of products over time. (4) is the best available measure of the market value of all final goods and services produced during a period.

To analyze the statements regarding why economists use gross domestic product (GDP) to measure a nation's total output, let's evaluate each statement: 1. **"is a reliable indicator of the social progress of a nation over time."** - This statement is somewhat misleading. While GDP can indicate economic growth, it does not necessarily reflect social progress, such as health, education, or income distribution. Therefore, this statement is not a strong reason for using GDP. 2. **"equals the sales value of all transactions conducted during a period and, thus, can be"** - This statement is incomplete, but it suggests that GDP represents the total sales value of goods and services. While GDP does measure the market value of final goods and services produced, it does not account for all transactions (like intermediate goods). Thus, this statement is not entirely accurate. 3. **"is unaffected by changes in the prices of products over time."** - This statement is incorrect. GDP can be affected by price changes, which is why economists differentiate between nominal GDP (measured at current prices) and real GDP (adjusted for inflation). Therefore, this is not a valid reason for using GDP. 4. **"is the best available measure of the market value of all final goods and services produced during a period."** - This statement is accurate. GDP is designed to measure the total market value of all final goods and services produced within a country during a specific period, making it a key indicator of economic performance. In summary, the most valid reason for using GDP as a measure of a nation's total output is statement (4). Statements (1), (2), and (3) either misrepresent GDP or are incomplete.