Human, in particular, are vulnerable to virus infection. what is the benefit of this devastation and why? 1. lowering human-population. 2. evolution. 3. increasing viral population. 4. none.

This is a very vague question. Beneficial to whom or what?
To earth/ecosystem? If we are talking about a system, then we need to define what is a good system. To me, a good system is a stable system, that is robust to perturbation. Diversity introduced by virus contributes to the robustness.
To human civilization, none most likely. I don't think lowering population is beneficial in the long run, so strike that out. Diversity is caused by mutation, while some virus can cause mutation, most do not. The selection process posed by virus actually decrease the diversity, as obviously only the fit ones are likely to survive and make offspring.  
TBH, this is kind of a dumb question. Cannot believe I wasted 5 minutes answering this.

 Answer 4: NONE While viruses can cause illness and even death in humans, they don't offer any inherent benefit, and the statement framing it as "devastation" is inaccurate.

In case of a viral infection, as the infection spreads there could more cases mortality thereby lowering human-population. Over a period of time, humans evolve to adapt to this infection which forces the virus to further evolve. This evolution increases the viral population. Considering this hypothesis as a possible scenario, it could be fare enough to state that all the 3 options are true. 
However, during the breakout of a viral infection there may have been several industries that have seen a boost in its economy which is seldom discussed.     

Viral infections in human is vital for creating a "Selection pressure" in a way promoting evolution. It can very well be envisaged based on the presence of retroviral elements in the human genome. This in a way creates a more resilient population that can withstand the infection if it happens in future.  

Evolution because in many cases, virus infections can result in human diversity by either promoting divergence or killing population groups that are less resistant to infections. For example, in many viral attacks, indigenous populations suffer more than native individuals. This is worst when viruses carried by humans or other animals migrate to new populations, which can significantly contribute to population decline. 

Lowering human population. Because there is no cure for viral infection. Eg. Millions of people died in developing countries because of HIV infection, millions of children lost their parents and forced to live in poverty, miserable life and death. Other recent devastating experience is Covid-19 infection. It is a cause for death of several millions people across in the world. In both cases there is no curative medicine. So what?......