What term would best describe a property of being extremely dry?

The term that best describes a property of being extremely dry is "arid." Arid regions or climates are characterized by very low rainfall, resulting in conditions that lack moisture and are often inhospitable to most forms of vegetation and wildlife. This term is commonly used in climatology and geography to refer to desert-like environments or areas that receive less than 250 millimeters (about 10 inches) of precipitation per year.

The other terms do not relate to dryness. "Arduous" refers to something that is difficult or strenuous, often related to physical effort. "Apt" describes something that is appropriate or suitable in the context. "Apprehensive" refers to a feeling of anxiety or fear about something that may happen in the future. Therefore, in the context of describing a property of being extremely dry, "arid" is the most accurate and relevant choice.