Explanation:

- The Historical School of law developed as a reaction primarily to the ideas of Natural Law School, which emphasized universal, rational principles above local traditions and customs.

- It also emerged in the context of events like the French Revolution, where natural law ideas were applied in radical reforms, sometimes with disruptive results. This led to a backlash valuing tradition and historical development.

- Option 1: French Revolution — Partially correct; the revolution highlighted excesses of abstract natural law, prompting a reaction.

- Option 2: Natural Law School — Correct; the main reaction was against the abstract, universal approach of natural law thinkers.

- Option 3: Both (a) and (b) — This is the most complete answer since the Historical School was a response to both the revolution's practical application and the philosophical ideas of the Natural Law School.

- Option 4: None of these — Incorrect; options (a) and (b) are both relevant.

Option 3: Both (a) and (b) is the correct answer.