Remember that this particular mutation in cystic fibrosis gene destroys a restriction enzyme site. if you did not know about the destroyed restriction site, what about the fragment patterns on your gel would indicate to you that a restriction enzyme site had been destroyed?

If a restriction site is altered, a higher molecular weight band will be observed.

Restriction enzymes are proteins synthesized by bacteria to protect itself against foreign DNA. These enzymes cut DNA at specific places.

This enzyme is used to detect mutations in the genome, particularly those at the sites of action of these restriction enzymes. A healthy subject is compared with the patient who is supposed to have cystic fibrosis. If their DNA migration is different, it means that their DNA is different and that there is a mutation at the restriction sites.

Restriction enzyme cleaves DNA into fragments at specific sites called as recognition sites. These enzymes are usually produced by bacteria to protect themselves against virus. Now they are also used in biotechnology and genetic engineering.

Here, cystic fibrosis gene sequence has a restriction site which is identified by restriction enzyme and cut into DNA segments of specific size.  If this site is destroyed due to a mutation, the DNA would not be cleaved like usual and hence the resulting band on gel will be bigger in size. The difference between control fragment patterns and sample fragment patterns can help to detect a mutation.  

the answer is d (apex) good luck : ))