Which one should I prefer for Android App Development??
1.Udacity
2.Udemy
3.Coursera
4.Other

Neither.
Just pick up a framework of your choice and start tinkering.

Just think of an idea for an app and start writing.

Google and ye shall find if you get stuck ;)

Despite what others have said, Udemy has helped me a lot. Just make sure to weed out all the shitty instructors, and once you’re comfortable with the topic, build something yourself from scratch.

I watched in the past some course from Udemy and other sites. Besides it I also thinkered on my own code and wrote myself.

A lot of stuff in the tutorials was not covered or they only showed their way, besides multiple options. Heck, some just read down a Wikipedia Articel 😡.

My suggestions is to watch tutorials, but also code yourself and take you're own way. Nobody can teach you the 100% best way, because you need sometimes to adapt you're code to the enivorement 😉