Was definitely Drogon and was definitely "drakaris" that she shouted.
https://youtu.be/AQtGZYEvFhA?t=4m16s
Besides that her three Dragons have always had distinct colours. Drogon is the black one, Viserion yellow, Rhaegal green, though they all have red mixed in.
Besides that Drogon is the only one she's ever rode. He's the biggest and baddest and it would have been weird for her not to take him into her first major battle.