Option A is correct. 'Beautifully' means 'in a beautiful manner'. Thus, it has the required meaning and so, it is the correct answer.
Option B is incorrect. The word 'beautifuler' does not exist in standard English. Thus, it is incorrect.
Option C is incorrect. The word 'beautifulless' does not exist in standard English. Thus, it is incorrect.
Option D is incorrect. The word 'beautifulness' does not exist in standard English. Thus, it is incorrect.