Assertion (A): Scientists can cut apart and paste together DNA molecules at will, regardless of the source of the molecules. Reason (R): DNA fragments can be manipulated using restriction endonucleases and DNA ligases.

The correct answer is B because both statements are scientifically accurate, but the Reason does not fully explain the Assertion. Assertion A is true because genetic engineering allows scientists to combine DNA from entirely different species, such as bacteria and humans, due to the universal nature of the DNA structure. Reason R is also true because restriction endonucleases act as molecular scissors to cut DNA at specific sites, while DNA ligases act as molecular glue to join fragments together. However, B is the correct option because the ability to manipulate DNA at will regardless of the source is primarily due to the chemical uniformity and universality of the DNA molecule itself. The tools mentioned in the Reason are the means to achieve the manipulation, but they do not explain why DNA from different sources can be successfully pasted together.