In which of the following books did Montesquieu propose a division of…

Concept: The doctrine of separation of powers holds that the three core functions of the state — making law (legislature), enforcing law (executive) and interpreting law (judiciary) — should rest in three distinct organs, so that no single organ concentrates enough authority to act arbitrarily. Its classical formulation is credited to the French Enlightenment jurist Charles-Louis de Secondat, Baron de Montesquieu, who derived it from his study of comparative constitutions and of the English system of his day.

Application: Montesquieu set out that tripartite division in De l’esprit des lois (1748), published in English as The Spirit of the Laws. In Book XI, Chapter 6 of that treatise he argues that liberty is lost when legislative and executive power are joined in the same person or body, and again when judicial power is not kept separate from the other two. That chapter is the source of the legislature–executive–judiciary scheme the question describes.

Cross-check (his other writings):

• Lettres Persanes (Persian Letters, 1721) — an epistolary satire in which two fictional Persian travellers comment on French society, religion and politics; a literary work, not a constitutional treatise.

• Mes Pensées (My Thoughts) — a posthumously assembled compilation of the author’s private notebooks, jottings and drafts, published long after his death; a personal miscellany rather than a systematic statement of doctrine.

• Because the tripartite scheme is set out in exactly one of the listed titles, the aggregate choices "More than one of the above" and "None of the above" do not apply.

Result: The tripartite division of powers among legislature, executive and judiciary was proposed in The Spirit of the Laws (1748). Its influence runs through the United States Constitution of 1787 and, in a diluted form, through the Indian Constitution, which distributes functions among the three organs (e.g. Article 50, separating the judiciary from the executive) without adopting a rigid American-style separation.