Translation:Where is the bus?
If your answer was "where is the public bus" then that's technically correct, but unless you need to specify that it's public you'd leave it out.
As somebody has mentioned, in (Mainland) China "公交车" is a common synonym for "公共汽车". This word is not used in Taiwan, we prefer to use "公車". Besides, a borrowing word "巴士" is also common. You can use them interchangeably.