Final answer:
An 'Inode' is not a valid file type on Linux; it is an underlying data structure in the filesystem. Whereas, 'Socket' and 'Softlink' are valid types of files used for IPC and referencing other files respectively.
Step-by-step explanation:
The question asks about file types on Linux and you need to identify which of the listed options is not a valid file type. In Linux, a Socket is a special file type used for IPC (Inter-Process Communication), a Softlink or symbolic link is a type of file that is a reference to another file or directory, and an Inode is a data structure on a filesystem that stores all the information about a file apart from its name and actual data content. Therefore, 'Inode' is not a file type but rather an underlying mechanism of the filesystem. So the correct answer is option 3) Inode, making it not a valid file type on Linux.