The correct answer is C, which indicates that widening spreads on the bank's issued debt and credit default swap are the strongest warning signals for potential liquidity risk at the bank. This is because wider spreads suggest a loss of market confidence in the bank, leading to a higher cost of funding. This can make it more difficult for the bank to raise the necessary funds to meet its obligations, thus increasing its liquidity risk.

Option A, a decrease in stock price of the bank's peers but not in the bank's own stock price, is not a strong indicator of liquidity risk for the bank itself. A more relevant early warning indicator would be a decrease in the bank's own stock price relative to its peers.

Option B, an increase in credit lines received from other financial institutions, is not a warning signal for liquidity risk. In fact, a decrease in available credit lines would be problematic as it could limit the bank's ability to access funds.

Option D, significant asset growth funded by an increase in stable liabilities, is not a warning signal for liquidity risk. On the contrary, rapid asset growth funded by volatile liabilities would be more concerning as it could lead to liquidity issues if the bank needs to sell those assets to meet its obligations and the market conditions are unfavorable.

Therefore, the consultant should consider the widening spreads on the bank's issued debt and credit default swap as the strongest warning signal for potential liquidity risk. This information is crucial for assessing the bank's liquidity risk profile and taking appropriate measures to mitigate it.