Which of the following best describes a casket that is impervious to external influence?

A casket that is impervious to external influence is best described as hermetically sealed. This term indicates that the casket is airtight, creating a barrier that prevents any external elements—such as air, moisture, or contaminants—from entering the interior. This level of sealing is crucial in protecting the contents from deterioration caused by environmental factors.

While protective casing refers to any covering that serves to protect its contents, it does not specifically denote the complete sealing that is necessary to be considered impervious. Weatherproof suggests resistance to moisture and other weather conditions but does not imply total protection from all external influences. Durable indicates the overall strength and longevity of the casket materials but lacks the specific implication of airtight sealing. Thus, hermetically sealed is the most appropriate description, clearly denoting the casket's capability to be completely isolated from outside factors.