a) twenty-first
My father was born on the twenty-first of.
1. Come in. I'm not sleeping. I never sleep in the afternoon. I'm just having a rest.
2. Max isn’t here. He is working in the garden. He usually works in his garden on weekends.
3. What is (your mother doing now? – She is cooking dinner. Does your mother cook every day?
4. Where is your sister? – She is helping mother to clean the flat. She always help mother with cleaning.
5. Hello! Where are you going? – Nowhere in particular. I'm just taking a walk. I sometimes take a walk after dinner.
Объяснение:
a) twenty-first
Форм "oneth" и "firth" не существует, "twenty-one" не подходит в данном случае, потому что нужно порядковое числительное.