I'm 21 years old, 5'5, 107lbs and a 32A. Will 400cc Silicone implants give me a full C cup?

Hi. To answer this question I would need to see photos of your chest/breasts including breast width measurements. Different people can achieve different results and cup size with the same implants - it depends on your chest wall size, breast size/position before the surgery as to what cup size and whether you get true cleavage with a particular implant.
The best things is to ask your surgeon to show you photos with similar body
frame and similar size implants to the ones you are thinking of to get a
realisitic idea of what to expect.
Try to find someone with a Vectra 3D imaging machine in your area to simulate
your breast augmenation result (see the link below) - in a recent article (Oct
2013) of Plastic& Reconstructive Surgery Journal (Plast. Reconstr. Surg.132:
810, 2013.), the main Plastic Surgery Journal worldwide, the 3D imaging VECTRA
machine was found to be beneficial for patients considering breast augmentation
surgery.

The article concluded that patients prefer a center that offers
three-dimensional imaging technology; they feel that the simulation is very
accurate and helps them greatly in choosing the implant; and that if they could
go back in time, they would choose the same implant again.

All the best with your decision. Regards Dr Charles Cope

Thank you for the question. Unfortunately, online consultants will not be able to provide you with specific enough advice to be truly helpful. Given your questions and concerns, you'll be best off spending additional time with your plastic surgeon communicating your goals carefully. Keep in mind, that there is no reliable correlation between the breast implant size/profile use and specific cup size achieved.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you're looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural”  or  "full C cup”  etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
 The use of computer imaging technology may also be very helpful during this communication process.

3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.
I hope this (and the attached link) help.