Explanation:

- Option 1: Effective Demand

- This is the correct answer.

- Keynesian theory centers around effective demand.

- Employment levels are determined by the total demand for goods and services.

- Higher effective demand leads to higher production and employment.

- Option 2: Supply

- Keynesian theory is more demand-focused.

- Supply doesn’t primarily determine employment levels in this theory.

- Option 3: Production Efficiency

- Efficiency affects costs but isn't central in determining employment in Keynesian theory.

- The theory emphasizes aggregate demand's role over production efficiency.

- Option 4: None of the above

- Incorrect, as effective demand is central to Keynesian employment theory.