A cipher that replaces plaintext symbols with other symbols.

• A. Substitution Cipher
• B. Transposition Cipher
• C. Caesar Cipher
• D. Polyalphabetic Cipher

Answer: (A)

Substitution is about replacing each plaintext symbol with a different symbol to produce ciphertext. In a substitution cipher, there is a fixed mapping from plaintext characters to ciphertext characters, so every symbol in the message is replaced by one other symbol, keeping the length the same. This contrasts with a transposition cipher, which reorders the same symbols without changing them. A Caesar cipher is a specific substitution that shifts every letter by a fixed amount, while a polyalphabetic cipher uses multiple substitution alphabets to vary the mapping. So describing a cipher that replaces plaintext symbols with other symbols points to substitution cipher as the correct idea.