(a) "The number of questions that you ask is never ending."
This sentence is correct with minor formatting adjustments.
(b) "All the food that you have ordered for your friends is going waste."
This sentence is correct with minor correction.
(c) "Most of the birds will be migrating to safer places within winter."
This sentence is incorrect due to the incorrect preposition.
(d) "I find it difficult to paying attention to his constant complaints."
This sentence is incorrect due to incorrect verb usage.
Sentences (a) and (b) are correctly structured. Therefore, the correct answer is (A) a and b.
Among scientists, the discovery of the double helix structure of DNA and the genetic code it incorporates is widely regarded to be one of the most significant scientific discovery of the twentieth century.