Which of the following is true about "reserva" wines in Portugal?

The statement that "reserva" wines must reflect the characteristics of the grape variety used is accurate. In Portugal, the designation of "reserva" indicates a higher quality wine that must meet specific aging requirements and quality standards that align with the characteristics typical of the grape varieties involved. This expectation ensures that the wine showcases the distinctive flavors, aromas, and qualities of the grapes from which it is made, allowing consumers to appreciate the varietal traits and the terroir of the region.

Regarding other options, while "reserva" wines can be blends, they are not required to be. Furthermore, "reserva" does not pertain to non-vintage wines; in fact, it often indicates specific vintage wines that have been aged longer. Additionally, "reserva" wines are typically aged in more traditional oak barrels rather than being confined to stainless steel, which can enhance their complexity and depth.