Bacteria protect themselves from viruses by fragmenting viral DNA upon entry with _____.
A. Methylase
B. Endonucleases
C. Ligases
D. Exonucleases.

Hint: To protect towards a phage attack, bacteria have advanced lots of immune systems. For example, whilst a bacterium with an immune system called CRISPR-Cas encounters a phage, the system creates a ‘memory’ of the invader with the aid of using a small snippet of the phage’s genetic material.

Complete answer:
To answer this question, first, we need to know about how bacteria protect themselves from viruses. A bacterium makes use of a restriction enzyme to protect towards bacterial viruses referred to as bacteriophages, or phages. When a phage infects a bacterium, it inserts its DNA into the bacterial cellular in order that it is probably replicated. The limit enzyme prevents replication of the phage DNA with the aid of reducing it into many pieces. Restriction enzymes had been named for his or her capacity to restrict, or limit, the quantity of traces of bacteriophage that may infect a bacterium.
Now, let us find the solution from the option.
Methylases include methyl groups to adenine or cytosine bases in the reputation sequence, that's as a consequence changed and guarded from the endonuclease. It is not directly related to protection of bacteria from viruses.

Restriction enzyme, additionally referred to as limit endonuclease, a protein produced with the aid of using micro organism that cleaves DNA at particular regions alongside the molecule. In the bacterial cellular, limit enzymes cleave outer DNA, as a consequence casting off infecting organisms. A bacterium makes use of a limit enzyme to shield in opposition to bacterial viruses referred to as bacteriophages, or phages.

DNA ligases play an essential role in preserving genomic integrity with the aid of using becoming a member of breaks within the phosphodiester spine of DNA that arise at some point of replication and recombination, and resulting from DNA harm and its repair. It isn't always related to protection of microorganisms from viruses.

 Exonucleases hydrolyze phosphodiester bonds in DNA from a loose end, play essential cell roles in DNA repair, genetic recombination and mutation avoidance in all organisms. It is not associated with protection of micro organisms.

Hence, the correct answer is option (B)

Note: When a phage infects a bacterium, it inserts its DNA into the bacterial cell in order that it is probably replicated. The limit enzyme prevents replication of the phage DNA with the aid of reducing it into many pieces. Restriction enzymes had been named for their capacity to restrict, or limit, the quantity of traces of bacteriophage that may infect a bacterium.