List the following investments in order of most to least liquid?

Savings,money under the mattress, CD, and stocks

There is not really enough information to answer this question with complete accuracy.

money in most cases is the most liquid but not in every case. Ask the folks in Mexico or Ecuador about their money under the matress right after the government did a devaluation. It became virtually worthless. Ask folks holding U S dollars about the loss of value day after day. That is not exactly illiquidity but not all that much removed either.

savings can become very illiquid especially if the government declares a bank holiday. It happens more frequently than one would wish. They may also become illiquid if the bank goes belly up. One might not have access to the savings for quite some time.

CDs. These can often be traded in after markets especially if they are purchased through a broker making them more liquid than might be imagined.

Stocks. Indeed most stocks can be traded any time the stock markets are open. But then there are times when the government closes the stock markets for one reason or another making stocks impossible to trade. Also the exchanges may halt trading in a stock for one reason or another. There is also the matter of the 3 business day settlement period.

money, savings, stocks, CD

Money under the mattress (because its yours)
savings accounts
CDs (comes with early withdrawal penaties)
stocks (could take a day or so to sell at a good price and get your dividend check)