Final answer:
The Supreme Court ruling that allowed inmates to assist each other in preparing legal documents is Johnson v. Avery. Option C.
Step-by-step explanation:
The correct answer to the question regarding which court case ruling found that prison authorities cannot prohibit inmates from aiding other inmates in preparing legal documents is c) Johnson v. Avery.
This Supreme Court case determined that inmates must be allowed to help each other with legal paperwork, particularly in situations where the prison does not provide adequate legal assistance to inmates.
This ruling upheld the rights of inmates ensuring access to the courts and the necessary means to challenge their convictions or conditions of confinement.
In contrast to the case in question, Miranda v. Arizona, which required that suspects must be "read their rights", the Johnson v. Avery case was specifically concerned with the rights of inmates to legally assist one another.
The decision in Gideon v. Wainwright also relates by ensuring the right to legal counsel for those who could not afford it, but in the context of a proper trial setting, not within prison walls regarding legal document preparation.
So Option c is the correct .