Question

the former Prime Minister ______ after a prolonged illness

1.passed off
2.passed into
3.passed away
4.passed on

Answer 1

the former prime minister passed off after a prolonged illness

Answer 2

The former Prime Minister passed away after a prolonged illness.

The correct answer is passed away as its meaning is closer to long term or end of life.

While passed off could mean slept or dozed off, passed into mean crossing something like a tunnel and passed on would mean to hand over something to other.

Thus among all the options available, passed away fits perfectly into the sentence.