It is quite common that most of the engineering background candidates make cross-domain shift and prefer a safer bet and some of the subjects from humanities or literature of languages comes on their radar.
Public Administration is a very good subject; but, we would suggest you to reconsider if it is a random choice. If you have never been in touch with it, how Public Administration has got preference over other optional subjects available?
In recent years, despite being most desirable subject, Public Administration has seen very average performance and now, when you have to pick only one optional subject, you should spend some time to find out what suits you the best in current environment.
The following article would help you in getting some insight about optional subject selection: