The correct answer is Bank Q. A liquidity crisis could arise if repo creditors become concerned about a bank's solvency and decide not to renew their positions. If a significant number of creditors choose not to renew, the bank may be unable to raise sufficient cash through other means on short notice, potentially leading to a crisis. The bank might then be forced to sell its assets quickly to buyers who know it needs to sell in a hurry, which can lead to a fire sale. This situation supports using the proportion of assets covered by repos as an indicator of liquidity risk. Additionally, low prices in a fire sale could decrease the market valuation of securities not sold, reducing the amount of cash that could be raised through repurchase agreements collateralized by those securities. The vulnerability to a liquidity crisis is directly related to the proportion of assets a bank has pledged as collateral. Bank Q is most vulnerable as it has the largest dependence on short-term repo financing, with the highest percentage of its assets pledged as collateral among the four banks.