Option B is correct because the suffix -ous turns the noun into an adjective, giving it the meaning 'full of'. In the sentence, "Robert is a mischievous boy", the adjective 'mischievous' modifies the noun boy i.e. Robert. Option A is incorrect because the letter f is dropped while adding the suffix -ous to it, and v is added in its place. Mischiefic and mischieful do not exist in the dictionary, they're not used in Standard English, hence options C and D are incorrect.