Consider the following sentences:
- Prisons are the best options for serial killers
- Prisons are the best option for serial killers
- Prisons are a necessary evil
- Prisons are necessary evils
Are all these correct? Why/why not?
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Prisons are the best options for serial killers
Wrong in most cases. Unless you are talking about a specific set of prisons that you, or someone, gets to choose from, prisons are not separate options. The concept of prisons, or the prison system (consisting of a collection of prisons) is a single option.
Prisons are the best option for serial killers
This is ok because you are referring to the collection of all prisons as a singular object, but normally you would instead say "Prison is the best option for serial killers."
Prisons are a necessary evil
Sounds ok even though it's probably not perfectly grammatical. Again, normally you would use the singular case: "Prison is a necessary evil." In the latter case, "prison" is treated more like a concept than a collection of buildings, which is more appropriate here.
Prisons are necessary evils
This is fine but see my comment above for the more common usage.