Short answer: You can do either or mix and match through your test!
If you start off in rising trot, then feel half way though a movement you would have better control in sitting, then do so, as long as you don't compromise the horse's way of going.
Vice versa as well, if you feel you want to do the two 10m circles or half circles in sitting trot, but then find the horse gets tense, gently go up to rising. It makes absolutely no difference to the marks the judge will give you if you sit or rise, what will make the difference is if the horse becomes tense/tight over the back or changes rhythm, tempo, etc., because you are sitting. You will certainly see your marks drop if this happens!
So, whilst you can, if you want to do most of it in rising, do so. Of course you will have to develop sitting trot to make the transitions as smooth as possible, so if you don't do much sitting, start practising at home. Have a nice soft (but forward) trot going and sit for a stride or two. If you or your horse gets tense, go back to rising. Constantly do this so that you both gradually build up more and more strides. You have all the time in the world. Sit for a few strides longer in and out of your transitions. If you feel yourself tense up, forget the transition and just go back to rising.
I have been asked if the judge will score higher for riders that sit more (the assumption being they are 'better' riders). Answer is no! (I've asked!). However, as you both develop, you may find it easier to get the best results if you are sitting, especially in the lateral movements.
It is not about being a 'better' rider if you can sit...it's about knowing the stage of development of your horse and yourself and it's about showing the judge the harmony between you both. If you insist on sitting but your horse gets choppy and tense because you are also tense, or gripping hard, this will not produce a good mark. If you feel soft and relaxed in sitting trot, but the horse is clearly unhappy, check it's back/saddle/etc. If you feel soft and relaxed and more connected whilst sitting and your horse responds more positively to you and the work, yes, you will get better marks, so it's something to aim for.
Sitting trot is not compulsory until you get to Medium, and even then, you can still do the Med trot in rising.