It is here to test your recognition of and distinction between singular and plural forms. This might be what is called a 'trick question'.
this one got me too. keeping us "on our toes" - English idiom for being alert
There is a group of English nouns that have irregular plural and instead of using the regular '-s' construction, mark plural differently(or don't do it at all :P ) – one of these methods is the so-called umlaut (vowel alteration) which happens to be the case with the noun 'man', so it has 'men' in plural. Others in this group of nouns are for example 'woman – women', 'tooth – teeth' and so on.