In modern ex-situ conservation strategies, the gametes of...

Cryopreservation is an ex-situ conservation strategy where gametes, seeds, or tissues of threatened species are preserved in viable and fertile conditions for extended periods by storing them at ultra-low temperatures, typically in liquid nitrogen at $-196^{\circ}\text{C}$.

Micropropagation is a tissue culture technique used for rapid vegetative multiplication of plants.

Somatic hybridization involves the fusion of isolated protoplasts from two different varieties of plants to produce a hybrid.

Biofortification is the breeding of crops with higher levels of vitamins, minerals, or proteins to improve public health.

Answer: Cryopreservation