In recent years, conservation strategies for threatened species...

Ex-situ conservation involves the protection of biological diversity outside their natural habitats.

Cryopreservation of gametes is an advanced ex-situ conservation method where gametes of threatened species are preserved in viable and fertile conditions for long periods using liquid nitrogen at $-196^{\circ}\text{C}$.

Biodiversity hotspots, sacred groves, and biosphere reserves are all examples of in-situ conservation strategies, where species are protected within their natural habitats.

Answer: Cryopreservation of gametes