5. The plane took off when all the passeng had already started heir seat belts.

В данном предложении необходимо вставить подходящую по смыслу фразу из предложенных:

1. had already been starting
2. had already started

passeng необходимо исправить на passengers, had уже есть в предложении, подходит первый вариант, он грамматически верен и подходит по смыслу к предложению.

The plane took off when all the passengers had already been starting heir seat belts.

В данном контексте более подходящим вариантом будет: passengers had already started putting on their seat belts.

Ответ: had already started putting on their