Option A is correct. 'good' is an adjective of quality. Also, 'a good book' is a grammatically correct phrase. Thus, it is correct.
Option B is incorrect. 'the worst' starts with an article and there is also an article before the blank. Two articles cannot be placed adjacent to each other and so ,using 'the worst' will make the sentence grammatically incorrect.
Option C is incorrect. 'worst' is an adjective of quality but, it is in the superlative degree. Adjectives of superlative degree are always preceded by a definite article(the). The sentence, however, contains an indefinite article(a). Thus, using 'worst' will make the sentence grammatically incorrect.
Option D is incorrect. 'very' is an adjective of degree and not an adjective of quality. Thus, it cannot be the answer.