"बिल्ली अपना पानी पीती है।"
अनुवाद:The cat drinks its water.
That answer is wrong for three reasons. Firstly we must use drinks with an -s here. For more info on using -s read this.
Secondly we must use an article before cat, it must either be a cat, or the cat. Just cat on its own is not correct
Finally it is not standard English to use "her" in this particular sentence. Her is generally only used when referring to something a female person owns: her house, her child, her face. An English speaker would probably use her when talking about their own cat (if it was female) for example "look at my cat, she's drinking her milk", but for a general cat people would use "its". Here are a few examples.