C is correct. Information availability (e.g., active financial news media or information regarding trading activity and traded companies) and financial disclosure should promote or increase market efficiency. 

A is incorrect. Arbitrage is a set of transactions that produces riskless profits. Arbitrageurs are traders who engage in such trades to benefit from pricing discrepancies (inefficiencies) in markets. Such trading activity contributes to market efficiency. If arbitrage opportunities increase, it means that there are either more pricing discrepancies or fewer arbitrageurs (or both), and this situation will lead to a reduction in market efficiency. 

B is incorrect. A decrease in trading activity can cause or accentuate other market imperfections that impede market efficiency.

B is correct. A momentum anomaly occurs when securities that have experienced high short-term returns continue to generate higher returns in subsequent periods. Therefore, if a trader can obtain persistent abnormal returns by adopting an investment strategy that purchases stocks that have recently experienced high returns, then he or she is exploiting a momentum anomaly. 

C is incorrect. The overreaction effect is a pricing anomaly that occurs when investors overreact to the release of unexpected public information, inflating (depressing) stock prices of companies releasing good (bad) information.

 A is incorrect. Data mining is not a market anomaly but a type of process that could be used to discover statistically significant price patterns.