For the GMAT Official Practice platform, answer explanations are only provided with the 90 free practice questions, not the Practice Exams. The Practice Exams are intended to mirror the real exam and experience, so test takers can become familiar with the question format and pace required to finish the exam.
This information is provided on our GMAT Official Starter Kit + Practice Exams 1 & 2 (Free) at https://www.mba.com/us/store/store-catalog/gmat-preparation/gmat-official-starter-kit-practice-exams-1-and-2-free.aspx and listed for the additional Practice Exams as well.