5. An experimental ground is a place (scientists / can / test out / their new inventions).
В данном случае, необходимо использовать слово where, так как речь идёт о месте.
Предложение с вставленным словом: An experimental ground is a place where scientists can test out their new inventions.
Ответ: An experimental ground is a place where scientists can test out their new inventions.