The correct answer for the theorist who developed the theory of `Legal Positivism` is John Austin. However, let`s examine each option to have a better understanding:

Option 1: T.H. Green - T.H. Green was a philosopher and political theorist. He is known for his work on ethics and political philosophy but is not directly associated with the development of Legal Positivism.

Option 2: Thomas Hobbes - Thomas Hobbes was a philosopher who is best known for his political philosophy, particularly the idea of the social contract. While Hobbes` work influenced many legal and political theories, he did not formulate the theory of Legal Positivism.

Option 3: John Austin - John Austin is the correct answer. He is widely recognized as the founder of Legal Positivism. Austin`s work emphasized the importance of law as a command issued by a sovereign authority.

Option 4: Patrick Devlin - Patrick Devlin was a British judge and legal philosopher. While Devlin made significant contributions to legal philosophy, he is not credited with developing the theory of Legal Positivism.

In conclusion, the correct answer is option 3, John Austin, as he is the theorist who developed the theory of `Legal Positivism.`