Explanation:

- Marginal Efficiency of Capital (Option 1): This is the expected rate of return on an investment, compared to its cost. Higher returns encourage more investments.

- Interest Rate (Option 2): This represents the cost of borrowing money. Lower interest rates reduce borrowing costs, encouraging investments.

- Both (Option 3): Both factors are crucial for investment decisions. The marginal efficiency of capital is about potential profits, while interest rates affect borrowing costs. INVESTORS use both to gauge viability.

- None of these (Option 4): This option implies other unspecified factors might be more relevant, which is unlikely, given the context.

Correct Answer: Option 3 - Both (a) and (b)