The correct option is D steadily / steady
Adverb of manner is used to tell the way or how something is done. Adverbs of manner are usually placed after the main verb or after the object. 'Steady' is an adjective and adding 'ly' will modify it. The adjective is something which describes the noun. In the second sentence, 'steady' describes the 'improvement'. Thus, option D is the correct answer.