Which of the following encryption methods is known to be unbreakable?

A One-Time Pad uses a keystream string of bits that is generated completely at random that is used only once. Because it is used only once it is considered unbreakable.

The following answers are incorrect:

Symmetric ciphers. This is incorrect because a Symmetric Cipher is created by substitution and transposition. They can and have been broken

DES codebooks. This is incorrect because Data Encryption Standard (DES) has been broken, it was replaced by Advanced Encryption Standard (AES).

Elliptic Curve Cryptography. This is incorrect because Elliptic Curve Cryptography or ECC is typically used on wireless devices such as cellular phones that have small processors. Because of the lack of processing power the keys used at often small. The smaller the key, the easier it is considered to be breakable. Also, the technology has not been around long enough or tested thourough enough to be considered truly unbreakable.