(A security analyst uses a polyalphabetic substitution cipher with a keyword of YELLOW to encrypt...

A polyalphabetic substitution cipher uses multiple substitution alphabets rather than a single fixed mapping. The classic cipher that uses a keyword to select shifting alphabets across the message is the Vigenère cipher. In Vigenère, each plaintext letter is shifted by an amount determined by the corresponding key letter (repeating the keyword as needed). For example, a keyword like “YELLOW” is aligned under the plaintext; each key character defines a Caesar shift (A=0, B=1, …) applied to the plaintext character, producing ciphertext. This rotation of alphabets across positions makes Vigenère more resistant to simple frequency analysis than monoalphabetic substitution, because the same plaintext letter may encrypt to different ciphertext letters depending on its position relative to the key. The Pigpen cipher is a symbol substitution cipher, Caesar is monoalphabetic with a single shift, and Playfair is a digraph substitution cipher using a 5×5 key square, not the repeating-key polyalphabetic method described. Therefore, the correct cipher is Vigenère.