In the given sentence, 'He is' suggests that the sentence is giving the subject a quality. Thus, the answer must contain an adjective of manner to describe the subject. Further, there must be an adjective of degree present to show that the subject is too busy to have regular meals.
Option A is correct. 'such a busy man' contains an adjective of manner 'busy' that describes the man. Further, it also contains an adjective of degree 'such' that shows that the subject is too busy to have regular meals. Thus, it is the correct answer.
Option B is incorrect. 'such busy man', although contains both, an adjective of manner and degree, it does not contain an indefinite article. according to rules of usage, an indefinite article is used before every singular pronoun or plural pronouns that act as one group. Thus, it is grammatically incorrect.
Option C is incorrect. 'so busy man' contains an adjective of manner, but it doe not contain an adjective of degree. 'so' is an adjective of manner and not degree. Further, it does not contain an indefinite article . Thus, it is grammatically incorrect.
Option D is incorrect. 'a so busy man' is incorrect because an article must always come before the noun, but in this case, the article 'a' comes before the adjective 'so'. Thus, the answer is grammatically incorrect.