Option C is correct because the suffix -ish is used to form adjectives to describe the manner or characteristic of something, in this case, of the boy. For example, in the sentence, "The young man had a boyish charm", the adjective 'boyish' modifies the noun 'charm' by attributing the characteristic of a boy to the man. The words 'boyful', 'boyed' and 'boyly' simply do not exist in the English dictionary, they're not used in Standard English. Hence, options A, B and D are incorrect.