Explanation

The correct answer to the question regarding the superlative degree of "costly" is B) "costliest." To understand why "costliest" is the appropriate choice, we need to explore the rules governing the formation of the comparative and superlative forms of adjectives in English.

The adjective "costly" is a one-syllable word with an 'y' ending, which typically converts to 'iest' when forming the superlative degree. This means we add "cost" to the superlative suffix 'liest,' resulting in "costliest." The superlative degree is used to indicate the highest degree of a quality among three or more items, and in this case, "costliest" means the item that is the most expensive among others.

Let's look at the other options for clarity:

- A) "Costier" is the comparative form, used to compare two items.
- C) "Coastlyest" is not a valid word and likely a misspelling.
- D) "Most costliest" is incorrect because "costliest" already encapsulates the superlative form, making the use of "most" redundant.

In summary, "costliest" is the correct superlative form of "costly," following standard grammar rules for adjective modification.