The correct answer is option 4: "spend a lot of time inside the host`s cells."

This option explains that viruses are difficult to kill because they spend a significant amount of time inside the host`s cells. When a virus infects a host, it hijacks the host`s cellular machinery to reproduce and spread. This allows the virus to evade the immune system and continue to replicate and cause harm.

Option 1 suggests that viruses consist of a tough protein coat, which is partially true. Many viruses do have a protein coat, called a capsid, which helps protect the viral genetic material. However, it is not the sole reason why viruses are difficult to kill.

Option 2 mentions that viruses are very small in size, which is true. Due to their small size, viruses can easily evade the immune system and enter host cells. However, small size alone does not make them difficult to kill.

Option 3 states that viruses lack cellular structure. This is true, as viruses are acellular and cannot carry out metabolic processes on their own. However, this lack of cellular structure is not the primary reason why it is difficult to kill viruses.

In summary, while options 1, 2, and 3 partly explain certain characteristics of viruses, option 4