Correct choice is (a) 2.5
The best explanation: In constant voltage drop model at forward bias diode can be replaced as a cell and in reverse bias diode can be avoided by considering the terminals are open.
Since first diode is forward biased and second diode reverse bias, we can ignore the second diode. Drop across the first diode is 0.5V and hence net voltage at output is 3-0.5=2.5V.