Can one mutual fund provide enough diversification?

As things stand, my portfolio has grown one investment at a time and now holds several stocks and funds. As things stand, it looks diversified because there are many line items, but I am not sure whether the holdings actually represent different companies, industries, and asset classes. Someone else in my family is relying on how I handle this choice, so liquidity, taxes, and flexibility matter more than they would if I were deciding only for myself.