1. No, it isn't a plural. It's one person or animal. In fact. 'their' isn't always plural - it can be used to refer to a singular subject. Nowadays, 'their' is the accepted possessive adjective to refer to 'someone', 'anybody', 'nobody', 'a person' 'whoever', or any other singular subject whose gender is unknown. We don't know whether this 'someone' is male or female, so we use 'their'. 2. Сan I use the words: "own", "his" or "its" instead "their" ? Can you use 'own'? No. The word 'own' is not a possessive form. You can add 'own' if you want - Someone is proud of their own work - but it can't replace 'their'. Can you use 'his'? Only if you know that the subject is male. Can you use 'its'? No, because 'its' doesn't fit with 'someone'. 'Their' is the only word that sounds right here. It's quite common to use 'they', 'them' and 'their' with a singular meaning, and this is actually a very useful way of phrasing things when you don't know, or it doesn't matter, whether the subject is male or female. For example . "I don't know who the last person was who used this shower, but they left their soap and towel there'. If you scroll down the articles on this site, you'll find one on this subject.

Ah! The old 'singular their' debate. Once upon a time, a long time ago, it was perfectly acceptable to use THEIR in a singular situation when the gender did not matter, or you didn't know it. Then a few grammarians got all upset and started a fashion for HIS or HER in those sort of situations. Thankfully the pendulum is swinging back! It is becoming acceptable once again to throw away that awful HIS or HER structure and go with THEIR ! Thank goodness! I always use THEIR, or re-word a sentence. In fact, I do ANYTHING to avoid HIS OR HER. Why write 3 words when you can write just one!

There is a good blog on the subject in the Oxford English Dictionary. http://blog.oxforddictionaries.com/2012/06/he-or-she-versus-they/

If you say "someone is proud of his work" that makes sense. Their is used by people as a replacement for him/her when the gender is not known. It depends what side of the grammatical debate you fall on. Old school traditionalists argue that words like someone, everybody, a person should always have his or her used with them as they are singular pronouns. "Anyone wishing to buy beer should have his or her identification ready." They are not wrong. Alternatively people do say "Could the owner of a blue Toyota please make their way to the parking lot?" It could be "Could the owner of a blue Toyota please make his or her way to the parking lot?" FACEBOOK uses Their in place of he or she when the gender is not known. It is Sarah's birthday wish them a happy birthday, It's Frank's Birthday write on their wall. So and so updated their status. Basically his or her and ITS have been replaced with THEIR so often in this case that it has become accepted. You can;t USE IT, because the phrase says Some ONE, so we know it's an entity and not an inanimate object "someTHING could be proud of its work."

Their is used when the thing mentioned is belonging to or associated with the person (cat). In this case the cat made a mess which is referred to as work. This means that the work belongs to the cat because the cat created it, so it becomes their work. But you could aslo use His or Her depening on it's gender. If you dont know you can always use It's.