The opportunity cost of capital refers to the -

Options

net present value of the investment.

Return that is foregone by investing in a project.

Required investment in a project.

Future value of the investments cash flows.

From the options given the best one is this one:

Return that is foregone by investing in a project.

Opportunity cost is simply what you could have gained from other options. So the opportunity cost of going for holidays to Cyprus is missing on Malta during the same time (for example)