Final answer:
The only unbreakable method of sending secret messages is the one-time pad in cryptography.
Step-by-step explanation:
In the field of cryptography, there are several methods used to send secret messages. Out of the four methods mentioned, only one is considered unbreakable, and that is the one-time pad.
The one-time pad is a symmetric encryption technique where a random key is used to encrypt the message. This key is used only once and is as long as the message itself. Since the key is truly random and used only once, it provides perfect security.
The other methods mentioned, such as transposition cipher, substitution cipher, and RSA encryption, can be broken with enough computational power or the use of certain algorithms.