The correct answer is Monoalphabetic cipher. It uses one alphabet shifted three places.
Answers Polyalphabetic cipher and Transposition cipher are incorrect because in answer a,
multiple alphabets
are used and in answer Transposition cipher, the letters of the message are transposed.
Answer Code group is incorrect because code groups deal with words
and phrases and ciphers deal with bits or letters.

The essential security principles of confidentiality, integrity, and availability are referred to as the CIA Triad.

All of the items listed can help secure faxes except printing a sensitive document banner, which actually encourages people to look at the document.