The correct answer is C. Productivity.
Productivity is the measure of the efficiency at which inputs are converted into outputs. It is a key determinant of market size because it affects the cost of production and the prices of goods and services. A higher level of productivity means that businesses can produce more goods and services with the same amount of inputs, which leads to lower costs and lower prices. This makes it more affordable for consumers to buy goods and services, which in turn leads to an increase in demand and market size.
The other options are not as important determinants of market size. Monetary spread is the difference between the interest rates on loans and deposits. It affects the cost of borrowing money, but it has a relatively small impact on market size. Opportunities for investors are important for attracting investment, but they do not directly affect the size of the market. Savings are important for financing investment, but they also do not directly affect the size of the market.
In conclusion, productivity is the most important determinant of market size. It affects the cost of production and the prices of goods and services, which in turn affects demand and market size.