4. 2) What is this? - This is a key;
3) What are these? - These are tissues;
4) What is this? - This is a watch;
5) What is this? - This is a camera;
6) What are these? - These are pens;
7) What are these? - These are dictionaries.
3. Не моё дело, конечно, но я, если Вы не возражаете, также подправлю и третье упражнение.
Apple - apples; person - persons (или менее формально - people); a diary - diaries; fox - foxes. Остальные, вроде как, написаны верно.