In option D the suffix -en transforms the noun to an adjective, meaning 'made of'. In the sentence, "The silken dress looked pretty on her", the adjective 'silken' modifies the noun 'dress', saying that 'the dress was made of silk'. Hence option D is correct. 'Silkful', 'silked' and 'silkly' are not words, they do not exist in the English dictionary, hence options A, B and C are incorrect.