Explanation:

In cryptography, the process of rearranging the sequence of letters in a message is done using transposition ciphers.

Transposition Ciphers:
Transposition ciphers are a type of encryption method that involves rearranging the letters or characters of a message without changing the actual letters themselves. This rearrangement can be done in various ways, such as reversing the order, rearranging based on a specific pattern, or shifting the positions of the letters.

Advantages of Transposition Ciphers:
Transposition ciphers offer certain advantages in cryptography. Some of them are:

1. Increased Security: Transposition ciphers provide an additional layer of security by rearranging the letters, making it more difficult for unauthorized individuals to decipher the original message.

2. Compatibility with Other Ciphers: Transposition ciphers can be used in combination with other encryption methods, such as substitution ciphers, to enhance the overall security of the message.

3. Flexibility: Transposition ciphers allow for a wide range of possibilities in terms of rearranging the letters. This flexibility provides the option to adapt the encryption method based on specific requirements or preferences.

Comparison with Substitution Ciphers:
While transposition ciphers involve rearranging the sequence of letters, substitution ciphers replace the letters with other letters or characters. In substitution ciphers, each letter is substituted with a different letter or symbol based on a predetermined key.

Transposition ciphers, on the other hand, do not replace the letters but instead change their positions within the message. This means that the original letters remain the same, but their order is rearranged according to the chosen transposition method.

Conclusion:
In summary, the correct answer is option 'A', transposition ciphers. These ciphers rearrange the sequence of letters in a message without changing the letters themselves, providing an additional layer of security in cryptography.