The correct answer is: A. chmod u + r, g + r, o – x letter
The chmod command is used to change the permissions of a file. The first argument is the mode, which is a string of three or four characters. The first character specifies the permissions for the owner of the file, the second character specifies the permissions for the group that owns the file, and the third character specifies the permissions for everyone else. The fourth character is optional and is used to specify special permissions.
The u character stands for the owner of the file, the g character stands for the group that owns the file, and the o character stands for everyone else. The + character means to add the permission, and the - character means to remove the permission. The r character stands for read permission, the w character stands for write permission, and the x character stands for execute permission.
So, the command chmod u + r, g + r, o - x letter will add read permission to the owner and group of the file, and remove execute permission for everyone else.
The other options are incorrect because they do not assign only read permission to all three categories of the file. Option B, chmod ugo = r letter, would assign read permission to the owner, group, and everyone else. Option C, chmod a - rw letter, would remove read, write, and execute permission from everyone. Option D, chmod go + r letter, would add read permission to the group and everyone else.