Final answer:
Investors who believe in the efficient market hypothesis would choose an index fund, as it is designed to mimic the market's performance and aligns with the EMH principle that consistently outperforming the market is not possible.
Step-by-step explanation:
Investors who strongly believe in the efficient market hypothesis (EMH) would be most likely to select an index fund. According to the EMH, all known information is already factored into stock prices, and it is impossible to consistently outperform the market average through expert stock selection or market timing. An index fund is designed to replicate the performance of a specific market index, such as the S&P 500, which provides broad market exposure and diversification, aligning with the EMH belief that an investment portfolio should reflect the market's overall performance rather than attempt to outguess it.