Final answer:
The 1st Amendment clause that establishes a separation between church and state is the Establishment Clause (A). It prevents government endorsement of religion and promotes neutrality toward religious beliefs.
Step-by-step explanation:
The clause in the 1st Amendment that sets up a wall separating church and state is A. Establishment Clause. This clause declares that Congress "shall make no law respecting an establishment of religion," effectively preventing the government from endorsing a state-sponsored religion, as well as favoring some religious beliefs over others or religion over non-religion. The Establishment Clause is often interpreted as creating a "separation" between church and state. On the other hand, the Free Exercise Clause, also part of the 1st Amendment, allows individuals the freedom to practice their religion without government interference, thus safeguarding religious freedoms alongside the Establishment Clause.