B is correct.
A liquidity crisis could materialize if repo creditors become nervous about a bank’s solvency and choose not to renew their positions. If enough creditors choose not to renew, the bank could likely be unable to raise sufficient cash by other means on such short notice, thereby precipitating a crisis. The bank may therefore be forced to sell its assets in a hurry to buyers that know it needs to sell quickly. This leads to the potential for a fire sale and supports using the proportion of assets covered by repos as a signal of liquidity risk. Also, low prices recorded in a fire sale could lower the market valuation of securities not sold, and thus reduce the amount of cash that could be raised through repurchase agreements collateralized by those securities. Overall, this vulnerability is directly related to the proportion of assets a bank has pledged as collateral.
Bank Q is most vulnerable since it has the largest dependence on short-term repo financing (i.e. the highest percentage of its assets out of the four banks is pledged as collateral).