The correct answer is Option A.
The given sentence means to say that the story is so good that it cannot be true.
Thus Option B- 'only good' is the incorrect option because it does not really express the degree of how good the story is.
'So' as an adverb of degree is always followed by 'that'. The correct usage would, therefore, be 'so good that...' Hence option C is not the correct answer.
'Too good' is incomplete and incorrect as the expression of degree using 'too' is always followed by 'to be...' Therefore Option D is incorrect.
Option A satisfies all the conditions- it expresses the degree of how good the story is. It is; therefore, the correct option.