Which term describes something that is very large in size or quantity?

The term "voluminous" is the correct choice for describing something that is very large in size or quantity. This word comes from the root "volume," which refers to the amount of space an object occupies. "Voluminous" specifically conveys the idea of great bulk or extent, making it suitable for describing anything from a large book filled with pages to a substantial amount of material or substance.

In contrast, "somber" refers to a mood that is dark or gloomy and does not relate to size or quantity. "Turbulent" pertains to a state of disturbance or disorder, often used in contexts related to turbulent weather or chaotic situations, which again doesn't apply to size. Lastly, "squalid" describes conditions that are extremely dirty or unpleasant, focusing on quality rather than size. Thus, "voluminous" is the most appropriate term for indicating vast dimensions.