There isn't a specific set of books officially titled "Homi Bhabha Exam Books" for 6th standard. The Homi Bhabha Centre for Science Education (HBCSE) doesn't publish specific textbooks for a particular exam at the 6th-grade level. Their focus is primarily on higher-level science Olympiads and training programs.
Sixth-grade students preparing for science Olympiads or competitive exams might use standard science textbooks prescribed by their school board or other general preparation books covering science concepts for that age group. These will vary depending on the curriculum and the specific exam.