Final answer:
The superlative degree of an adjective is formed by adding -est to the end of the word.
Step-by-step explanation:
When forming the superlative degree of an adjective, what is commonly added to the end of the word?
The correct answer is C. -est. When forming the superlative degree of an adjective, the suffix -est is added to the end of the word.
For example, 'tall' is a positive adjective, 'taller' is the comparative degree, and 'tallest' is the superlative degree.
Learn more about superlative degree of adjectives