Liquidity is important because investors must be able to buy or sell shares without causing a significant movement in the market price. A liquid security has enough trading volume, market depth, and active participation to absorb transactions efficiently. This is especially important for institutional investors, but it also matters for retail investors because illiquid shares can have wider bid-ask spreads and greater price impact. Option D is partly related, but it is narrower and focuses only on retail trading costs rather than the broader market-quality issue. Option B may be indirectly affected by investor confidence, but it is not the main liquidity-analysis point. Option C is incorrect because high liquidity generally improves market efficiency. Option A best captures the concept.