Explanation:

- Option 1: Keeton

- Keeton is known for contributions to the study of jurisprudence.

- However, he is not specifically noted for sub-dividing material sources into legal and historical sources.

- Option 2: Salmond

- Salmond is a well-regarded legal scholar.

- Salmond did sub-divide material sources into legal and historical sources, as per his works on jurisprudence.

- This is the correct answer.

- Option 3: Paton

- Paton has made significant contributions to legal theory.

- He is not the one specifically associated with the sub-division of material sources into these categories.

- Option 4: Gray

- Gray is known for his work on legal realism.

- He did not focus on the subdivision mentioned.