The correct answer is option 4: a vitrified liquid. Glass is not a crystalline solid because it does not have a regular, repeating atomic arrangement like crystalline solids. It is also not an ionic solid because it does not consist of positively and negatively charged ions held together by electrostatic forces. Glass is not an elastic solid because it does not exhibit elastic behavior, that is, it does not return to its original shape when deformed.

Instead, glass is considered a vitrified liquid. This means that it appears solid, but its atomic structure is more closely related to that of a liquid. The atoms or molecules in glass are arranged in a disordered manner, without any long-range order. Over time, glass may exhibit slight flow or deformation, indicating its liquid-like behavior. This is why old windows in historic buildings may be thicker at the bottom, as the glass has slowly flowed downward over time.

It`s worth mentioning that there is some debate among scientists regarding the exact nature of glass, and multiple models have been proposed to explain its behavior. However, the most widely accepted view is that glass is a vitrified liquid.