An analyst recommends that you invest in a portfolio made up of Bank of Montreal (BMO), Bank

Question:

An analyst recommends that you invest in a portfolio made up of Bank of Montreal (BMO), Bank of Nova Scotia (BNS), Canadian Imperial Bank of Commerce (CM), and Royal Bank (RY). Why would it not useful in diversification?

Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...