Option 1: A bacterium which infects animals

This option is incorrect. Bacteriophages specifically infect bacteria, not animals.

Option 2: A bacterium which breaks petroleum

This option is incorrect. Bacteriophages have no role in breaking down petroleum.

Option 3: A virus which lyses bacteria

This option is correct. Bacteriophages, also known as phages, are viruses that specifically infect and replicate within bacteria. They attach to the surface of bacteria and inject their genetic material, which then takes over the bacterial machinery to produce more phages. This eventually leads to the lysis, or bursting, of the bacterial cell and the release of new phages. Bacteriophages are being studied for their potential use in treating bacterial infections.

Option 4: A protozoan which feeds on bacteria

This option is incorrect. Bacteriophages are not protozoans, but viruses. They do not consume bacteria, but instead replicate within them and eventually cause their destruction.

In conclusion, a bacteriophage is a type of virus that infects bacteria and causes their lysis or destruction.