The correct answer is Saving-income ratio​.

• Level of savings:
  • ​Higher savings enable greater investment in capital stock.

• The Harrod Domar Model:
  • It is used in development economics to explain an economy's growth rate in terms of the level of saving and of capital.
  • The Harrod Domar Model suggests that the rate of economic growth depends on two things:
    • Level of Savings
    • Capital-Output Ratio
    • In simple form economic growth= \(Level of Savings \over Capital-Output Ratio\)
    • Level of savings:
      • ​Higher savings enable greater investment in capital stock.
    • The marginal efficiency of capital:
      • ​This refers to the productivity of investment, e.g. if machines costing $30 million increase output by $10 million.
      • The capital-output ratio is 3.
    • Depreciation:
      • ​Old capital wearing out.
• The Harrod Domar Model introduced a concept known as the warranted growth rate.
• This is the growth rate at which all saving is absorbed into investment. (e.g. $100 of saving = $100 of investment).

• According to the Harrod–Domar model, there are three sets of growth:
  • Warranted growth 
  • Actual growth 
  • The natural rate of growth
• The warranted growth rate is the rate of growth at which the economy does not expand indefinitely.
• Actual growth is the real rate increase in a country's GDP per year.
• Natural growth is the growth an economy requires to maintain full employment.