There are several online question-answer platforms and forums that can be highly beneficial for the revision of Mathematics Optional subject for UPSC exams. Here are some recommended platforms:

1. EduRev
EduRev is a comprehensive educational platform that offers a range of study materials, notes, and practice questions for various subjects, including Mathematics Optional. It provides a user-friendly interface with a vast collection of questions and answers, making it an excellent resource for revision.

2. Quora
Quora is a popular question-answer platform where users can ask questions and receive answers from a community of experts. It has a dedicated section for Mathematics where students can find and ask specific questions related to their optional subject. It is essential to verify the credibility of the answer providers before relying on the information provided.

3. Stack Exchange - Mathematics
Stack Exchange is a network of question-answer websites, and the Mathematics section is one of its most active communities. It allows users to ask questions and receive answers from a community of mathematicians and enthusiasts. The platform also encourages users to upvote the best answers, ensuring high-quality responses.

4. MathOverflow
MathOverflow is a platform specifically designed for research-level mathematics questions. While it may be more advanced than the UPSC syllabus, it can still be useful for students looking for in-depth explanations or discussions on specific topics.

5. Math StackExchange
Math StackExchange is another well-known platform for mathematics questions. It caters to a wide range of mathematical topics, including those covered in the UPSC syllabus. Users can ask questions and receive answers from a knowledgeable community that includes both professionals and enthusiasts.

6. UPSC Mathematics Optional Facebook Groups
Facebook groups dedicated to UPSC Mathematics Optional can be a valuable resource for revision. These groups often have experienced candidates or experts who can provide guidance, answer questions, and share relevant study materials. However, it is essential to verify the credibility of information shared within these groups.

In conclusion, these online question-answer platforms and forums can be highly beneficial for the revision of Mathematics Optional subject. However, it is crucial to verify the credibility of the information provided and use these platforms as a supplement to other study materials and resources.