1. A fall in the bank rate tends to increase the money supply because it makes borrowing cheaper for commercial banks. When the central bank (such as the Reserve Bank of India in India) reduces the bank rate, it lowers the interest rate at which commercial banks can borrow money from the central bank.  
2. As a result, commercial banks are more inclined to borrow additional funds at a lower cost.

    This, in turn, allows commercial banks to lend more money to businesses and consumers at lower interest rates, which encourages borrowing and spending. Increased borrowing and spending by businesses and consumers result in a higher money supply in the economy.

So, the reason for the answer is that a fall in the bank rate leads to increased lending and, consequently, an increase in the money supply.