We don't know from the word if which refers to a single or a plural thing. Using feel or feels tells us this. Which of them is French? - This suggests that only one of the people is French. Which of them are French? - This suggests that more than one is French. Sometimes we don't know if it is single or plural so we use the one that we think is most likely. We only find out when we get the answer.

This is called synesis or notional agreement, if you want to look into it further. There is lots of debate on grammar forums about this, and most of the experts and people who work in the field would say that it's perfectly correct and completely logical for nominally singular things to take plural verb forms if they are being referred to as a plurality. It is generally people who were taught these rules at school and simply repeat them that say it's wrong. This shows a fundamental misunderstanding of how language works. In the case of computers, it seems they are in the latter camp. Whether that's because the programmers think such 'rules' can actually be prescribed, or it's just really difficult to do the other way is a good question. Synesis relies on context and an understanding of the world, and I'm not sure you can model that on a machine. I suppose it's this ignorance that makes the algorithms so sure of themselves - so they do display some genuine human traits, after all) Out of interest, could you translate the following sentences into Russian and then get the bots to translate them back into English? Would be interesting to see what they do: The group is pretty informal - people join and leave all the time. The group are pretty informal - most members are really laid back. Regarding your sentence, if the people you're talking to know that you're referring to several people who you consider the 'most' calm, then it's perfectly correct. It may appear to break several grammar 'rules', but we do this all the time as it makes things easier to say and understand. Context should placed above all other things. I will continue to use synesis although it is quickly being eradicated online because of grammar checkers. You know the saying 'they're a lovely couple'? You see things like 'it's a nice couple', now. I don't know how that sounds to you but it sounds horrendous to me. You even see 'the police is ...' now, which was always an exception.

Because it's short for "which one of them feels..." meaning "which one feels..." meaning "who feels..."