How does copyright work on questionnaires with similar items?
Copyright law varies from place to place, so a valid answer might depend on where you are. But in general, it is a good idea to assume that copyright law applies to everything. Basically if you write something (or create in another medium) you own rights to it. Some places you may have to register the copyright as was true in the past in the US. Most places copyright is a matter of civil, not criminal, law. But I think there are exceptions. 

There are some caveats, however. Copyright doesn't prevent you from using something, only from copying it or obtaining a copy illicitly. There are a few exceptions to the general prohibition against copying, but they are a bit subtle. 

The first is fair use, but that is a subtle thing. While there are fair use exceptions for some uses, including research, they aren't unlimited exceptions and depend on a "proportional" standard that can only be adjudicated by agreement between parties or by a court in a lawsuit. (Note, this is a US view, things vary around the world). 

So, since you are doing research, you can depend on fair use up to a point, but that point isn't very clear. A few items out of a hundred is probably not an issue. How few? No-one can say in general. If the copyright holder objects, then you have a problem that will be expensive to solve. 

However, another caveat may be more important here. You can't really copyright things for which there is only one basic way to say or write. So lots of things in mathematics can't be copyrighted because the "expression" is more or less immutable. Copyright doesn't "protect" ideas, only the expression of ideas. So, if "The boss is kind and gentle" is the idea, then there are only a few ways that that can be said in a questionnaire, so it is unlikely anyone would have a basis for a copyright complaint. 

Unless you copy a lot of those kinds of things from a single source. Then the source is more likely to start to question your intent and might want to raise it in a lawsuit. My guess is that is pretty unlikely, but don't discount it altogether. You can get a bad reputation as a researcher for things that don't break any legal rules. 

But you also need to consider the question of plagiarism. If you copy a bit and don't cite the source then you have a different issue, though it, too, may be covered by the "one reasonable form of expression" idea. 

A claim of copyright can always be made. Courts, however, won't be supportive of frivolous claims, where only a small part was copied or the reasonable ways to express an idea are very limited. Therefore it is unlikely that a claim is likely to be made if things happen "by chance". A valid claim would need to depend on fairly clear evidence, especially given the research (partial) exception as stated in the law. 

See especially the section on the four considerations used to evaluate fair use claims in the link above. I think the third one is what you need to be most concerned with:


  the amount and substantiality of the portion used in relation to the copyrighted work as a whole;


But, of course, if you need to repeat a questionnaire in its entirety, say for reproducing the research, you can always ask the authors for permission. 

Moreover, if you reuse a questionnaire that you find in a copyrighted work that you have valid access to, there should be no issue as long as you don't reproduce that questionnaire in your own work. You just refer to the published questionnaire in your methodology section.