Need help in a project

I need help on one of my friend's project in his studies. Project is : Two 12 volt DC inputs named A and B given, if A input goes off then a buzzer and small LED is switched on and if A input comes back the buzzer and small LED is switched off. Please help with diagram so that I can tell perfectly. I am beginner in electronics

Why doesn't your friend post here?

Post the exact assignment, so we know what the professor wanted and not a 'filtered' version subject to misinterpretation.

I would use a relay. Power the buzzer and LED from battery B and drive the 'activation' signal from battery A. I am not going to tell you how to do it, but you can wire the relay up so that when battery A dies, the relay contacts change over to connect the power from battery B through to the parallel connected buzzer and LED. Be careful to put a current limiting resistor in series with the LED

As simon said, the simplest every way to do things is to use a relay. However, a more energy efficient way would be to use a transistor or two..
Why don't you use some online simulator (like partsim) to play around and test... it costs nothing, and is the best way to learn!

Come back here and ask us more specific questions, and i (we) 'll be more than happy to help