The most common measure of market potential of an economy is a country's GNI. GDP. PPP. CPI. APR.

To determine the most common measure of market potential of an economy, let's analyze each option:

1. GNI (Gross National Income): This measures the total income earned by a country's residents and businesses, including any income earned abroad. It reflects the overall economic strength and the income level of the population.

2. GDP (Gross Domestic Product): This measures the total value of all goods and services produced within a country’s borders in a specific time period. It is a key indicator of economic activity and market potential.

3. PPP (Purchasing Power Parity): This is a method of measuring the relative purchasing power of different countries' currencies over the same types of goods and services. It helps compare economic productivity and standards of living between countries.

4. CPI (Consumer Price Index): This measures the average change over time in the prices paid by consumers for a market basket of consumer goods and services. It is primarily used to measure inflation.

5. APR (Annual Percentage Rate): This represents the annual rate charged for borrowing or earned through an investment, and it is expressed as a percentage. It is not typically used to measure market potential of an economy.

Among these options, the most common measure of market potential of an economy is GDP (Gross Domestic Product). It provides a comprehensive snapshot of a country’s economic activity and is widely used to gauge the economic performance and potential of a country.