In a plasmid vector, the number of sites for a restriction endonuclease is present
(a) Twice
(b) Only once
(c)Any number
(d) Restriction site not needed

Hint: Since multiple copies of the same restriction site will digest the vector to produce various fragments of DNA so the number of sides should be less than two.

Complete answer:
In a plasmid vector, the number of sites for a restriction endonuclease is present only once. There may be multiple cloning sites in a plasmid vector made up of different types of restriction sites. But all of them will be repeated only once because multiple copies of the same restriction site will digest the vector to produce various fragments of DNA, after the action of the restriction enzyme. Due to this the factor will fail to serve its function of producing a recombinant DNA. This will cause the presence of only one site of restriction endonuclease.

Additional Information: -The restriction enzymes are found in bacteria and the sequences of DNA which they recognize and bind are called the restriction sites.
-The DNA cutting enzymes are restriction enzymes also known as restriction endonuclease.
-One or a few target sequence is recognized by each enzyme and they cut the DNA at or near those sequences
-Plasmid vectors are double-stranded circular DNA molecules that are small and with a bacterial replication origin.
-The plasmid vectors are capable of producing a very high level of replication and convenient restriction sites.
So, the correct answer is,'Only once'.

Note: -The plasmid vectors are designed to work in bacteria.
-Replication of the plasmid vector occurs along with their host cells.
-A vector is generally a vehicle that is used by the researchers in the laboratory to introduce DNA into a host cell.