Explanation:

- Foreign Direct Investment (FDI): Generally involves long-term investments like setting up businesses or acquisition of assets in a foreign country. It is considered relatively stable.

- External Commercial Borrowings (ECB): These are loans obtained from foreign lenders by a country or companies. They tend to have medium-level volatility due to interest rate fluctuations and economic conditions.

- Foreign Aid from ADB: Aid typically involves financial support for specific projects with relatively stable and predictable flows.

- Foreign Portfolio Investment (FPI): Consists of investing in foreign financial assets like stocks or bonds. It is highly volatile due to quick reaction to market conditions and investor sentiment.

- Correct Answer: FPI is indeed the most volatile flow in forex due to its sensitivity to short-term market movements and investor behavior.