Final answer:
The word "vein" in the context of a stratum is best matched with the word "seam," which refers to a layer of minerals or materials in the earth, similar to a vein.
Step-by-step explanation:
The word "vein" when used in the context of a stratum or layer is best matched by the option (a) seam. A seam is a layer of material, often referring to minerals like coal, that lies between layers of other materials, similar to how a vein refers to a distinct layer of minerals or rock within the earth.
In contrast, a river, mountain, and desert do not refer to layers within the earth and therefore, do not match the meaning of "vein" in the context of a stratum.