Final answer:
The El Gamal encryption algorithm is the one based on the Diffie-Hellman key agreement. It leverages discrete logarithm problems for secure key operations, unlike the other listed options which are based on different cryptographic principles.
Step-by-step explanation:
The encryption algorithm based on the Diffie-Hellman key agreement is C) El Gamal. The Diffie-Hellman key exchange is a method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols. El Gamal encryption is an asymmetric key algorithm that uses the principles of the Diffie-Hellman key agreement protocol for its public key operations. It leverages the difficulty of calculating discrete logarithms in a finite field, which is a hard problem that provides security for the algorithm.
Other options provided, such as Knapsack, International Data Encryption Algorithm (IDEA), and HAVAL, are not directly based on the Diffie-Hellman key agreement. Knapsack is an algorithm based on the knapsack problem, while IDEA is a symmetric key block cipher and HAVAL is a cryptographic hash function, none of which are based directly on Diffie-Hellman.