Possessive case

http://www.englishexercises.org/makeagame/viewgame.asp?id=2452

http://www.englisch-hilfen.de/en/exercises/nouns_articles/apostroph

https://www.ego4u.com/en/cram-up/grammar/nouns/exercises?04

https://engelsklaslokaal.nl/en/english-grammar-exercises/miscellaneous/possessives-s-of/

http://www.english-4u.de/poss_case_ex1.htm
https://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-4640.php