The most appropriate word to fill in the blank is:

Option 1: "contain"

The sentence should read: "Carbohydrates are so called because they contain carbon, oxygen, and hydrogen."

"Contain" is the correct word in this context because it accurately describes how carbohydrates are composed of or include carbon, oxygen, and hydrogen. The other options do not convey the intended meaning:

• Option 2: "Cover" suggests a different relationship between carbohydrates and these elements, and it doesn't accurately describe their composition.

• Option 3: "Create" doesn't fit the context because it doesn't describe the makeup of carbohydrates in terms of these elements.

• Option 4: "Codify" means to arrange or organize something according to a set of principles or rules, which is unrelated to the composition of carbohydrates.