The given sentence is a synthesis of two sentences that were combined to form a simple sentence with the expression 'too...to'. 'Too' in the underlined phrase means 'more than enough', it refers to the painting's beauty that is more than enough because it was painted by a mediocre artist. The sentence has the proper structure of 'too...to' hence the given sentence is correct, option D is correct. This is a simple sentence that contains the adverb 'too', when it is converted to the complex form, 'too' is replaced by 'so...that'. If we use option A, the sentence would need to be changed a little more by adding 'that' followed by a negative clause, as in "so beautiful that it could not have been painted by...", and we are not supposed to change this sentence to complex, hence option A is incorrect. Option B is incorrect as 'that' is not relevant here, it means 'as much as that', which does not complete the sentence, because it conveys a positive meaning whereas the sentence has a negative tone. (it's hard to believe that a mediocre artist would paint such a beautiful painting). Similarly, 'very' emphasizes the beauty of the painting, which is irrelevant to what the sentence states, 'very' is a positive adverb whereas 'too' has a negative tone, as it refers to an excess. Hence option C is incorrect.