In 1973 Bell and LaPadula created the first mathematical model of a multi-level security system.
The following answers are incorrect:
Diffie and Hellman. This is incorrect because Diffie and Hellman was involved with cryptography. Clark and Wilson. This is incorrect because Bell and LaPadula was the first model. The Clark-Wilson model came later, 1987. Gasser and Lipner. This is incorrect, it is a distractor. Bell and LaPadula was the first model.