Genetic engineering is possible because
A.The phenomenon of transduction in bacteria is well understood.
B.We can see DNA by an electron microscope.
C.We can cut DNA at specific sites by endonucleases like DNAse I.
D.Restriction endonucleases purified from bacteria can be used in vitro.

Hint:Genetic engineering means making changes in the original genome with the help of recombinant DNA to introduce desirable characters in the plant or animal which are naturally absent in them and remove such genes that are harmful to the plants and animals..

Complete answer:
-The process of genetic engineering is possible due to the presence of restriction endonuclease which helps us to modify the original DNA sequence.
-Restriction endonucleases are also known as restriction enzymes which can Identify and cut DNA only at certain sites known as restriction sites.
-Since the restriction enzymes can cut the DNA strands they are also known as molecular scissors.
-Restriction enzymes can be naturally obtained from bacterial cells.

The restriction endonuclease can be easily used to alter the DNA sequence hence a non-desirable sequence can be easily removed and a desirable sequence can be inserted to change the genetic arrangement of the organism to get the desirable phenotype, Hence the genetic material obtained from bacteria that can be used in vitro facilitates the genetic engineering. Therefore, this is the correct option.

Hence the correct answer is option ‘D’.

Note:The restriction sites of DNA obtained by restriction endonuclease are palindromes that mean the bases on two strands read the same when they are read in opposite directions. For example, if the sequence is GAATTC in one strand the sequence on another strand will be CTTAAG but when we read the sequence of another strand in the opposite direction we get the same sequence as the first strand.