Final answer:
The question deals with symmetric encryption using a block cipher, key, and initialization vector to encrypt the provided plaintext with a certain block size. The actual encryption mechanism is not provided, but typically an algorithm like AES could be used.
Step-by-step explanation:
The question involves the application of symmetric encryption using a block cipher, specifically one that seems to involve the use of an Initialization Vector (IV). In this case, the plaintext 'YORKTOWNSUNK' would be encrypted with the key 'NAVY' and the IV 'FROG', given a block size of 4 characters. The IV is typically used in cryptographic algorithms like Cipher Block Chaining (CBC) mode to ensure that identical blocks of plaintext will result in different ciphertext in separate encryption instances, thus enhancing security.
To encrypt the plaintext, each block of 4 characters would be processed along with the key and IV. The exact encryption algorithm isn't specified, but a common one that might be used is AES (Advanced Encryption Standard). Without specifying the encryption algorithm, however, we cannot provide a specific encrypted output. It is also worth mentioning that the processing of the last block may require padding if the plaintext does not evenly divide into blocks the size of the block cipher.