Under what condition are co-defendant statements that implicate a defendant generally admissible in criminal cases?

In criminal cases, statements made by a co-defendant that implicate another defendant are generally admissible when the co-defendant testifies and is subject to cross-examination. This is rooted in the Confrontation Clause of the Sixth Amendment, which guarantees a defendant the right to confront witnesses against them. When a co-defendant takes the stand, their statements become subject to scrutiny through cross-examination, allowing the defendant to challenge the credibility and reliability of those statements.

Additionally, this process helps ensure that the defendant can defend against the accusations made in the co-defendant's testimony, which upholds the fair trial rights of the defendant. Without the ability to cross-examine the witness, those statements may be seen as unreliable or unfairly prejudicial, therefore often excluded if the co-defendant does not testify.

The other scenarios outlined in the provided options do not provide the same level of reliability or protection for the defendant’s rights, making the admissibility of the statements contingent on the ability to cross-examine the individual making the accusations.