To solve the given question, we need to select the word that best fits the context of the sentence. The sentence is: "Not __________ metal is necessarily solid in its natural state."

Let's analyze the options:

1. each: "Each" refers to individual items within a group. In this context, "not each metal" could imply specific individual metals, but the sentence's meaning is intended to address a broader classification of metals.
2. every: "Every" implies all without exception. However, saying "not every metal" can sound counterintuitive, as "every" is usually used in inclusive contexts.
3. all (Correct Answer): "All" refers to the entirety of a group. Using "Not all metals are necessarily solid in their natural state" accurately conveys that while many metals may be solid, some are not, fulfilling the sentence's need to express a general exception rather than a definite rule.
4. any: "Not any metal" implies the absence of metal, whereas the sentence needs to suggest that metals may have states other than solid in some cases.

Therefore, the correct option is all. The sentence "Not all metal is necessarily solid in its natural state" correctly communicates that some metals might exist in states other than solid.