The efficient market explanation that fits Adam's request is the semi-strong form: security prices already reflect publicly available information. Mark should keep the response simple because Adam asked for a plain- language explanation, not a technical lecture on market microstructure. The point is that, in an efficient market, new information is incorporated into prices quickly, making it difficult for investors to consistently earn abnormal returns from information that the market already knows. Option B describes learning from past mistakes, which is not efficient market theory. Option C is too absolute; price history may be analyzed by technicians, but the theory focuses on information efficiency rather than saying past prices have no relation to anything. Option D describes behavioural variation among investors, not the core theory. AFP investment planning uses this concept to distinguish passive market exposure from active security selection. Study Guide focus: efficient markets, passive investing, market information, and investment philosophy.