Ethereal is the correct answer because it is the only option among the given choices that represents something intangible or not physical. To understand why ethereal is the correct choice, let's examine the meanings of the other options:

a) Concrete: Concrete refers to something that is physical, tangible, or existing in reality. It is the opposite of ethereal.

b) Actual: Actual means real or existing in fact, so it is similar to concrete in that it describes something that is tangible or physical.

c) Solid: Solid refers to something that is firm, stable, or not easily changed. While it can describe physical objects, it can also be used metaphorically to represent something reliable or dependable.

Given these meanings, only ethereal stands out as the option that represents something intangible, immaterial, or not physical. It is often used to describe things that are delicate, airy, or spiritual in nature.

In summary, the word ethereal is the correct choice because it represents something intangible or not physical, whereas the other options describe things that are tangible, physical, or existing in reality.