Prep Guru is one of the good apps available on Playstore.Unacademy is also another good app to prepare with.
Youth4 is.one of the best application for the preparation of BSNL JAO.It provide study material as well as mock tests easily.
The application which I found informative and helpful is BSNL JAO by Youth4work. DO give it a try. It provided me with study material as well mock tests. Every day new questions are asked making me aware of my strengths and weaknesses. Do try it, you'll learn a lot.