"In this study, cancerous and normal skin tissue samples were excised with a scalpel. Each skin sample was then placed into a sample tube along with solvent, shaken on a vortex and centrifuged. When the sample tubes came out of the centrifuge, there was a top layer and a bottom layer. For our purposes, we went on to use for further processing."

top layer

Explanation:

You will get two part while you shake the sample on vortex and centrifuge it.  Bottom part is called pellet and the top part is called supernatant. All the cellular materials like DNA are contained in Sup. Pellet contains tissue debris. That's why you need to move forward with that part (top) for further processing and ignore the bottom part.

i might be wrong but i think its a. let me know if i’m right and have a good rest of your day!