"The way conflicts and contrasts are resolved along with the theme of 'mistaken identities' make the Twelfth Night a successful comedy." Discuss.
Rather a complex essay title you have been given there, zoha. To begin you need to "unpack" its various parts so you can work out how you can respond to each section of the title. The question seems to be focussing on the resolution of all the confusion and problems in the play - how the love triangle is "solved", and how the various disguises are taken off and identities are re-established. Therefore one way to answer this question would be to consider whether, in fact, the play does resolve all of these conflicts and issues - is it just a "happy ending" kind of Shakesperian comedy or is Shakespeare trying to do something else instead?
I have recently answered a very similar question and I have put the link to the answer below, so have a look at that and see if it helps you in thinking through some of the ways that the "resolution" doesn't actually "resolve" some of the central conflicts. One additional point that you might want to think through is are we convinced as an audience by the ending? We have seen characters fall in love suddenly and inexplicably. Take Olivia's crush on Cesario, and then the way she (mistakenly) marries Sebastian very quickly. Are we convinced that marriage or love that is described throughout the play variously as a "plague" or a "sickness" or an "infection" will result in a happy marriage? Olivia has married Sebastian - a man she knows next to nothing about. Are these the kind of ingredients that will give us the happy ending Act V scene i seems to promise? We could ask the same question about Orsino and Viola - Orsino never "knows" the true Viola and yet they marry too.
Just some ideas - hope they help!