Brenda Lee is most concerned about the aspect of investing called "Return." Return refers to the amount of money an investment earns over a certain period of time. In this case, Brenda is considering investing in stocks because she knows that stocks have been earning about a 10% rate of interest over the last several years.
By focusing on the return, Brenda is interested in maximizing the potential growth of her investment. She wants to make sure that her $10,000 gift from her mother can earn a significant return in the stock market. This is why she is considering investing in stocks, as they have historically provided a higher rate of return compared to other investment options.
It's important to note that return is just one aspect of investing. Other factors like diversification, liquidity, and risk should also be considered when making investment decisions. However, based on the information provided, Brenda's primary concern is the potential return on her investment.