The correct answer is A. chmod ugo + x letter.
The chmod command is used to change the permissions of a file. The u, g, and o stand for user, group, and other, respectively. The + sign means to add permissions, and the x sign means to make the file executable. So, chmod ugo + x letter will add executable permission to the owner, group, and other users of the file letter.
Option B, chmod u + x letter, will only add executable permission to the owner of the file. Option C, chmod u + w letter, will only add write permission to the owner of the file. Option D, chmod g + x letter, will only add executable permission to the group of the file. Option E, None of the above, is incorrect because it does not specify which permissions will be added.