Final answer:
Before the Miranda ruling, the sole test for the admissibility of a confession was voluntariness.
Step-by-step explanation:
The sole test for the admissibility of a confession before the Miranda ruling was c) voluntariness. Prior to the Miranda v. Arizona case, confessions could be admitted as evidence as long as they were deemed voluntary. However, the Miranda decision established that confessions are only admissible if the suspect is made aware of their rights and waives those rights.