Final answer:
Umbrella seals are the seals that fit over the valve stem to prevent oil from entering the combustion chambers.
Step-by-step explanation:
The blank seals that fit over the valve stem to prevent oil from entering the combustion chambers through the valve guides are umbrella seals, also known as C) Umbrella seals. These seals are designed to keep oil from flowing down the valve stems and into the combustion chambers during the combustion process.