Opportunity Cost of Capital

Consider a portfolio manager whose mission is to maximize annual returns through investments in the bonds of Industrial-Property REITs. Which is the best opportunity cost investment for this manager?

A mutual fund that makes loans to multi-family rental REITs.

Blackstones Low-Volatility, Industrial REIT Mutual Fund, which purchases the bonds of REITs that manage industrial properties.

Blackstones Aggressive, Industrial REIT ETF, which makes equity investments in REITs that manage industrial properties. -- this one is incorrect

Vanguards REIT Industry mutual fund, which makes money by purchasing the bonds of a large number of REITs of all types.