What is the superlative form of the adjective costly?

If it's from study Island its Costliest

Explanation:

When comparing more than two things, you use a superlative. The superlative form of an adjective or adverb has either the word "most" or the ending "-est." Two-syllable adjectives that end with "y" form the superlative by dropping the "y" and adding "iest."