Explanation:

- Kingsley Davis: Renowned sociologist who specialized in demographic studies, but he did not define slavery in the manner described.

- L.T. Hobhouse: A British sociologist and political theorist. He emphasized the evolution of society's moral and ethical dimensions. Despite his work, the specific definition of a slave isn't typically attributed to him.

- Plato: Ancient Greek philosopher known for his works on political philosophy and ethics. He discussed topics such as justice and society but did not define slavery as described.

- Rosseau: Likely refers to Jean-Jacques Rousseau, an influential philosopher. He wrote extensively on topics of freedom and inequality but did not define slavery in those precise terms.

?? Correct Answer: L.T. Hobhouse

Based on the description, the definition aligns closely with Hobhouse's domain of work.