Which method is commonly used to store cryogenic liquids?

C is the correct choice because Dewar containers are specifically designed for the safe storage and transportation of cryogenic liquids. These containers are insulated and constructed to minimize heat transfer, which is crucial for maintaining the low temperatures required for cryogenic substances. Dewar containers can maintain the temperature of liquids like liquid nitrogen or liquid helium, allowing for safe storage without the risk of vaporization that would occur in less specialized containers.

In contrast, other options such as cylinders are typically used for gases under pressure and may not offer the necessary thermal insulation for cryogenic applications. Flammable containers could pose safety risks due to their inability to handle the extreme cold of cryogenic liquids. Lastly, wooden kegs are inappropriate for storing cryogenic materials as they cannot withstand the extreme conditions and may lead to hazardous situations. Thus, Dewar containers stand out as the safest and most efficient choice for cryogenic storage.