Which of the following is a purpose of mortgage funding?

The purpose of mortgage funding primarily revolves around facilitating the borrowing and lending of money for the purchase, refinance, or improvement of real estate. Choosing to refinance existing mortgages is significant because it allows homeowners to improve their financial situation by securing lower interest rates, reducing monthly payments, or accessing home equity for other financial needs. Refinancing can lead to significant savings over time or a shift to different loan terms that better fit the homeowner's current financial circumstances.

When it comes to the other options, they relate to uses of funds that fall outside the specific realm of mortgage funding. Securing personal loans typically involves different forms of credit evaluation and does not involve mortgage securities. Financing business start-ups involves using capital for operations and growth, which, again, is usually separate from real estate mortgages. Lastly, investing in stocks pertains to the trading of financial instruments rather than real estate transactions. Therefore, the focus of mortgage funding is specifically aligned with refinancing existing mortgages, making this the correct answer.