Which of the following is a recognized exception where the declarant must be unavailable?

The recognition of certain exceptions under the hearsay rule is crucial for understanding when a declarant's unavailability is necessary for admission of evidence. In the case of former testimony, this exception allows certain statements made under oath in prior proceedings to be admissible in current cases, but only when the declarant is unavailable to testify. This is rooted in the principle that the prior testimony was made in a context that provided some reliability or accountability, given that the witness previously had the opportunity to be cross-examined.

By contrast, affidavits from the same case do not require unavailability because they can be presented as part of the existing case's evidence without the same concerns that necessitate the unavailability requirement. As for expert witness statements not made in court, these do not meet the criteria of prior testimony that qualifies for hearsay exceptions, nor is the presence of an expert witness in the current case considered non-availability. Character evidence typically does not invoke the unavailability rule as it can often be introduced through various means that do not rely on past testimony under oath.

Thus, former testimony is validated as a hearsay exception with the condition of unavailability, reflecting a balance between the need for reliable evidence and the practicalities of trial procedures