The correct answer is B.

The majority shareholding of stock 'Y' by a single family significantly reduces the number of shares available for trading. In the context of stock markets, liquidity refers to the ability to quickly buy or sell a particular security without causing a significant change in its price. A stock is considered liquid if there are enough shares available in the market for trading. When a single entity, such as a family, holds the majority of a company’s shares, the number of shares available for trading in the market decreases. This reduction in available shares makes the stock less liquid, as there are fewer shares that can be bought or sold quickly without impacting the stock's price. Therefore, the majority shareholding of stock 'Y' decreases the number of shares available for trading, making it less suitable for inclusion in a portfolio that requires liquid stocks.

Choice A is incorrect. The majority shareholding of a company does not directly affect the profitability of the company. Profitability is determined by factors such as revenue, costs, and operational efficiency, which are independent of who holds the majority shares.

Choice C is incorrect. While transaction costs can be influenced by liquidity and trading volume, they are not directly affected by who holds the majority shares in a company. Transaction costs are typically determined by factors such as brokerage fees and bid-ask spreads.

Choice D is incorrect. As explained above, while the number of shares available for trading can be influenced by who holds the majority shares in a company (as it may limit supply), transaction costs are not directly affected by this factor.