The correct answer is A.
A sale of investment in marketable securities implies a source of funds. This is because when you sell an investment, you are essentially converting it into cash. This cash can then be used for a variety of purposes, such as paying down debt, investing in other assets, or simply spending it.
Option B is incorrect because an application of funds is when you use money to purchase something. In the case of selling an investment, you are not using money to purchase something, but rather you are converting it into cash.
Option C is incorrect because there is a flow of funds when you sell an investment. The funds flow from you to the buyer of the investment.
Option D is incorrect because the sale of an investment does imply a flow of funds.