Final answer:
A transcript of testimony at the preliminary hearing is admissible at trial if the witness is unavailable to testify.
Step-by-step explanation:
In a criminal trial, a transcript of testimony at the preliminary hearing is admissible at trial if the witness in question is unavailable to testify. This is because the Sixth Amendment guarantees the right to confront and cross-examine witnesses. However, it is important to note that hearsay, or testimony by one person about what another person has said, is generally inadmissible in a criminal trial unless it falls under specific exceptions, such as an admission of guilt by the defendant or a 'dying declaration' by a deceased person.