Step 1: List the clues in the question.
The material is a natural colloidal clay, it absorbs water, it swells 8 to 15 times its dry size, and it is used to seal farm ponds.
Step 2: Match the swelling clue to a known clay mineral.
Among common pond sealing materials, bentonite clay (mostly montmorillonite) is well known for this large swelling ratio when wetted, far more than ordinary clays.
Step 3: Rule out the synthetic options.
EPDM and butyl are manufactured rubber sheet liners, not colloidal clays, so they cannot be the "natural" material the question asks for.
Step 4: Rule out soil cement.
Soil cement is a compacted soil and cement mix, an engineered material, and it does not swell the way a colloidal clay does.
Final Answer:
Bentonite is the natural colloidal clay used to seal farm pond beds.
\[ \text{Answer: Bentonite} \]