The investment multiplier represents the overall increase in the national income resulting from an initial increase in investment. It is calculated using the formula:

Multiplier = {1}/{1 - MPC}

where MPC is the marginal propensity to consume.

If the marginal propensity to consume (MPC) is 0.8, then the value of the investment multiplier is:

\[Multiplier = {1}/{1 - 0.8} = {1}/{0.2} = 5

Therefore, the value of the investment multiplier is 5, corresponding to option (2).