Bank Q is the most vulnerable to a liquidity crisis because it relies most heavily on short-term repo financing and has the lowest proportion of unencumbered (not pledged) assets available to generate liquidity. To assess vulnerability, we can look at the ratio of "Pledged as collateral" to total "Owned" instruments, or the ratio of "Not pledged" (unencumbered assets that can be used to raise funds) to "Pledged" (representing the refinancing need):

• Bank P: 258 / 656 = 39.3% pledged (Not pledged to pledged ratio = 398 / 258 = 1.54)
• Bank Q: 472 / 750 = 62.9% pledged (Not pledged to pledged ratio = 278 / 472 = 0.59)
• Bank R: 139 / 339 = 41.0% pledged (Not pledged to pledged ratio = 200 / 139 = 1.44)
• Bank S: 209 / 835 = 25.0% pledged (Not pledged to pledged ratio = 626 / 209 = 3.00) Bank Q has the highest reliance on repo financing (62.9% of assets encumbered) and the lowest buffer of unencumbered assets relative to its short-term debt, making it most vulnerable if creditors refuse to roll over repos.