Q. By default, a Linux user belongs to which group?

Explanation by: Jatin Dubey

Users are organized in groups, each user belongs to at least one group and can belong to other groups. Belonging to a group gives you special access to the files and directories authorized for that group.
By default, a Linux user belongs to the same group as the user ID (UPG).
UPG or User Private Group is a unique group ID with the same name as the user ID.