One time pad is the correct option because it is the only technique that can provide information theoretic or theoretical unbreakability when used under strict conditions.

One time pad achieves this level of security only when the key is truly random, the key is at least as long as the plaintext, the key is kept completely secret, and the key is never reused.

One time pad is impractical for most real world systems because key generation and secure distribution are difficult for long messages, and this practical limitation is why it is not commonly used despite its theoretical strength.

Elliptic curve cryptography is based on hard mathematical problems and offers very strong practical security, but it is not information theoretically unbreakable because its security depends on computational assumptions and on the current limits of algorithms and computing power.

Single DES is insecure because its 56 bit key size is too small and it is vulnerable to brute force attacks, and the algorithm has been deprecated and replaced by stronger standards so it is not considered unbreakable and it is less likely to be correct on newer exams.

Symmetric ciphers as a class include many algorithms that are secure in practice when keys are well managed, but they are not universally unbreakable because their security is based on computational hardness and not on information theoretic proofs except for the one time pad.

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