What does the mount option ro dev mean if placed in the fourth field /etc/fstab file?

What does the mount option ro dev mean if placed in the fourth field /etc/fstab file?

• Mount the filesystem as read-only and permit device files
• Nothing, there is no dev mount option
• Mount the filesystem as read-only and don’t permit device files
• Nothing, the option is invalid because a space is not permitted between options