C.

1) had been sitting

2) had been working

3) had been chewing

4) had been dreaming

5) had been minding

6) had been wondering

7) had been disappearing

8) had been stealing

Объяснение:

Present Perfect Continuous образуется по схеме:

had been + V-ing

D.

1. Michael had been sitting in the garden for a few hours.

2. Michael was exhausted because he had been working long days.

3. Michael hasn't noticed that Rufus had been chewing on a slipper.

4. Rufus had been minding his own business till a shadow caught his attention.

5. Michael didn't notice the commotion because he had been dreaming about going on holiday.

6. For the last few weeks Michael had been wondering why Rufus's biscuits disappearing.

7. The dog biscuits had been disappearing because the cat had been stealing it.