Ответ:

2 They've built a lot of new hotels along the coast. - a lot of new hotels have been built along the coast

3 Someone has just booked the last seat on the plane. - the last seat on the plane has just been booked

4 Have they cleaned the swimming pool yet? - Has the swimming pool been cleaned yet?

5 A lot of people have recommended that activity holiday.  That activity holiday has been recommended by a lot of people

6 They haven't encouraged tourism in this part of the country.​ The tourism in this part of the country hasn't been encouraged by them

Объяснение:

На первое место ставится объект, над которым совершается действие. Далее глагол to be меняется по временам + глагол основной в третьей форме. В данных предложениях время Present Perfect, потому глагол to be ставим в "have/has been" и прибавляем основной глагол в третьей форме - delayed/ cleaned/ booked / recommended / encouraged. Все смысловые глаголы правильные, потому к ним добавляем окончание "ed" - это их третья форма