Here are the correct variants for each sentence:
The boy has already done his homework and now he is watching TV.
- c) has already done, is watching
They have been talking about Russian traditions since the beginning of the lesson.
- a) have been talking
This is the best conference I have ever been to.
- b) have ever been to
The postman usually comes at 9 in the morning. It is half past 9 now but he has not come yet.
- a) comes, has not come yet
They have painted the walls and they look nice.
- b) have painted, look
The economic situation in the world is already bad and it is getting worse.
- a) is getting
If you have further questions or need more practice, feel free to ask!