Mechanical Engineering is a popular optional subject chosen by many candidates in the UPSC examination. It is important to have access to high-quality study materials and online resources to effectively prepare for this subject. Fortunately, there are several resources available that can help candidates in their preparation. Here are some of them:

1. EduRev:
- EduRev is an online education platform that offers a wide range of study materials for various subjects, including Mechanical Engineering.
- It provides comprehensive notes, video lectures, practice questions, and previous year question papers for Mechanical Engineering.
- The platform also has a community of students and experts who can help in clarifying doubts and discussing important topics.

2. UPSC official website:
- The official website of UPSC provides a detailed syllabus for Mechanical Engineering optional subject.
- Candidates can download previous year question papers and sample papers from the website to get an idea about the exam pattern and the type of questions asked.

3. Online forums and discussion platforms:
- There are several online forums and discussion platforms where candidates can interact with fellow aspirants and experts in the field of Mechanical Engineering.
- These platforms provide a platform to ask questions, discuss important topics, and get valuable insights from experienced candidates and experts.

4. Online courses and video lectures:
- Many educational websites and YouTube channels offer online courses and video lectures specifically designed for Mechanical Engineering optional subject.
- These courses provide in-depth coverage of the syllabus, explanation of concepts, and problem-solving techniques.
- Candidates can also find recorded lectures from reputed coaching institutes on various topics of Mechanical Engineering.

5. Reference books:
- There are several reference books available in the market that cover the entire syllabus of Mechanical Engineering optional subject.
- Candidates can choose books authored by renowned authors and recommended by experts in the field.
- It is advisable to refer to multiple books to get a comprehensive understanding of the subject.

In conclusion, candidates preparing for the Mechanical Engineering optional subject in the UPSC examination can make use of online resources like EduRev, the official UPSC website, online forums, and discussion platforms, online courses and video lectures, as well as reference books to enhance their preparation. These resources provide a wealth of information, practice questions, and expert guidance, which can significantly contribute to a candidate's success in the examination.