The correct option is B Cecil says she has a very busy schedule tomorrow.
Adverbs of time usually go at the end of the sentence. Sometimes, they
can be placed in the front if we want to emphasize the time.
The given sentence does not contain the correct placement of the adverb 'tomorrow'. It gives the impression that the person, Cecil, will say tomorrow that she has a very busy schedule. Hence the sentence needs to be changed, making option A incorrect.
The adverb of time must be placed at the end of the sentence. Since option B has the adverb in the end, it is correct.
Option C is incorrect because the adverb must be placed at the end. In C it seems to modify the pronoun.
Option D is incorrect. Even though an adverb of time can be placed in the front, there are exceptions. Here the adverb seems to modify the sentence, implying that Cecil will state that she has a busy schedule only when tomorrow comes (in this case the tense should also be in future, not present). Not that she has a busy schedule the next day.