Option A is correct because '-able' forms an adjective, meaning, 'capable of causing laughter'. In "Miss Daisy had a laughable mannerism," the adjective 'laughable' modifies the noun 'mannerism'. The words 'laughful' and 'laughly' are not used in Standard English, hence, options B and D are incorrect. 'Laughingly' is an adverb, not an adjective. Hence, option C is also incorrect.