How is the efficient frontier determined?

It is made up of all assets from the opportunity set that maximizes your return.

It is the risk-return trade-off for every possible combination of all risky assets available.

It is the portfolio with the largest slope formed between combinations of a risk free asset and a risky portfolio.

It is made up of risky assets from the opportunity set that minimizes risk for any given return; or maximizes return for any given risk.