What is the superlative form of the adjective costly?

most costly

Explanation:

The superlative form would be taking the base and adding est, or adding most in front of it, depending on which is grammatically correct. In this case, it would be most 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."