2. An engineer is a person (profession / apply scientific principles to engines, machines and different constructions like bridges and buildings).
В данном случае, необходимо использовать слово who или that, так как речь идёт о человеке.
Предложение с вставленным словом: An engineer is a person who applies scientific principles to engines, machines and different constructions like bridges and buildings.
Ответ: An engineer is a person who applies scientific principles to engines, machines and different constructions like bridges and buildings.