https://www.usingenglish.com/quizzes/348.html
http://www.englishexercises.org/makeagame/viewgame.asp?id=4219
https://www.ego4u.com/en/cram-up/tests/relative-clauses-4
http://www.esl-lounge.com/student/grammar/4g31-advanced-relative-pronoun-gap-fill-quiz.php
http://www.esl-lounge.com/student/grammar/4g26-advanced-relative-pronouns-reordering.php
https://languageavenue.com/advanced-english/advanced-grammar-exercises/relative-clauses-1-advanced
scroll down to do the tasks: https://learnenglish.britishcouncil.org/en/english-grammar/clause-phrase-and-sentence/verb-patterns/relative-clauses
http://www.englisch-hilfen.de/en/exercises/pronouns/relative_pronouns2.htm