The most reliable starting point is Duolingo's official documentation for test-takers: The DET Handbook

Since you only have 20 seconds per question, timed practice is essential. Many online resources include built-in timers. Try to finish in 15 seconds or less to build a safety margin.

Duolingo provides official booklets for test preparation that include "Read and Complete" sections. DET Official Handbook (PDF)

The question format you're practicing for, officially known as "Read and Complete," is a cloze test designed to assess your ability to understand context, grammar, and vocabulary. This question type is one of the most challenging on the Duolingo English Test (DET), with a strict 20-second time limit. Duolingo does not release official PDFs of practice questions, and no third-party source provides an all-in-one, official compilation.