While I (25) _______ (sit) on Sarah's bed I noticed that her favourite posters were no longer on the walls.

Выполним задание в соответствии с ПРОТОКОЛОМ Е.

25) Необходимо образовать правильную форму глагола sit в прошедшем продолженном времени (Past Continuous), так как действие происходило в определенный момент в прошлом.

Правильная форма глагола sit в Past Continuous — was sitting.

Следовательно, предложение будет выглядеть так:

While I was sitting on Sarah's bed I noticed that her favourite posters were no longer on the walls.

Ответ: While I was sitting on Sarah's bed I noticed that her favourite posters were no longer on the walls.