What principle was established in the school-sponsored prayer case of Engel v. Vitale?

Based on the alternatives that were provided on the comments, the correc alternative is: separation of church and state.

The Engel v. Vitale case ruled to be unconstitutional for state officials to create school prayer or recitations in public schools because it violated the Establishment Clause in the First Amendment to the US Constitution.

Separation of church and state