If you are learning German at an advanced level (B2-C1), you have likely come across the highly popular exercise book "Grammatik aktiv B2-C1" (published by Cornelsen, authored by Ute Voß and Friederike Jin). This book is a staple for mastering complex German grammar through explanation and practice.
At B2-C1, you should aim to understand why an answer is correct, not just copy it. The Lösungen are a tool for verification, not a shortcut. grammatik aktiv b2-c1 losungen