Which of these words (engaged in, busy with, occupied by... or some other) should I use?
Most of their pastime is BUSY WITH testing and using mobile gadgets, applications and internet surfing.

1. Which of these words (engaged in, busy with, occupied by... or some other) should I use?
2. Is the sentence grammatically correct?

Question

Which of these words (engaged in, busy with, occupied by... or some other) should I use?
Most of their pastime is BUSY WITH testing and using mobile gadgets, applications and internet surfing.

1. Which of these words (engaged in, busy with, occupied by... or some other) should I use?
2. Is the sentence grammatically correct?

Answer

"occupied by", "taken up by"
Grammar is fine