Which one of the following is considered a necessary precondition by Richard Roll for markets to become efficient?

The correct answer is "Traders who make a living by 'beating the market' through trading"Key Points

 Roll's Critique for markets to become efficient:

• Roll's critique is an economic idea that suggests that it is impossible to create or observe a truly diversified market portfolio.
• This is an important idea because a truly diversified portfolio is one of the key variables of the capital asset pricing model (CAPM), which is a widely used tool among market analysts.
• According to Roll's critique, a true "market portfolio" would include every investment in every market, including commodities, collectibles, and virtually anything with marketable value.
• Roll's critique is a reminder of the fact that one can only diversify a portfolio so much, and that investors' attempts to understand and know the market as a whole are just attempts.

Hence, According to Richard Roll, for making a market efficient Traders who make a living by 'beating the market' through trading becomes an essential pre condition.