"चिड़िया सेब खाती है।"
अनुवाद:A bird eats the apple.
There are a few errors. If it was 'birds', then it would be "The birds eat the/an apple". Since it is plural, you use 'eat' and not 'eats', and you need to use an article (a, an, the). But this can't be the answer. If it was 'birds', the Hindi translation would be "चिड़ियाँ सेब खाती हैं।"
सेब can be either one (apple) or more (apples). In the first case, you need to use an article before 'apple'.
'A/The bird eats an/the apple'
In the second case, you can talk about apples in general (here you won't need any article) or some specific apples (here you need the before 'apples')
'A/The bird eats apples' or 'A/The bird eats the apples'