The correct answer is: A. Low P/E ratio effect
The low P/E ratio effect is a well-documented anomaly in the stock market that suggests that stocks with low P/E ratios tend to outperform stocks with high P/E ratios over the long run. This anomaly is consistent with the semi-strong form of market efficiency, which states that all publicly available information is reflected in stock prices.
The size effect is another well-documented anomaly in the stock market that suggests that small-cap stocks tend to outperform large-cap stocks over the long run. This anomaly is not consistent with the semi-strong form of market efficiency, as it suggests that there is information that is not reflected in stock prices.
The effect on the stock split is a phenomenon that suggests that stocks that split tend to outperform stocks that do not split. This phenomenon is not well-understood, but it is possible that it is due to investor psychology.
The weekend effect is a phenomenon that suggests that stocks tend to underperform on Mondays relative to other days of the week. This phenomenon is also not well-understood, but it is possible that it is due to trading patterns or liquidity effects.
In conclusion, the low P/E ratio effect is the only statement that provides evidence for the semi-strongly efficient form of the market.