Final answer:
The Lemon v. Kurtzman case established the Lemon test, setting a precedent for evaluating the separation of church and state by ensuring laws have a secular purpose and don't promote or inhibit religion.
Step-by-step explanation:
The court case that set an important precedent for determining whether or not a law was in violation of the US Constitution's separation of church and state is d) Lemon v. Kurtzman. In the Lemon v. Kurtzman decision of 1971, the Supreme Court established the Lemon test, which is used to assess whether government actions or laws are in accordance with the Establishment Clause of the First Amendment. The Lemon test has three criteria: the law must have a secular legislative purpose, must not have the primary effect of advancing or inhibiting religion, and must not result in an excessive government entanglement with religion. This ruling addressed whether state-funded support for teachers' salaries and materials in nonpublic, religious schools violated the Establishment Clause.