Ответ:

1. Economical (the other three words describe something that is expensive or costs a lot)
2. Identical (the other three words describe something that is not the same or different in some way)
3. Amateur (the other three words describe something that is related to professional activities or work)
4. Invention (the other three words describe the result or consequence of something)
5. Accept (the other three words describe the action of giving or distributing something)
6. Basic (the other three words describe something that is challenging or requires effort, while basic means simple or easy)