Answer:
Option C, Mass
Step-by-step explanation:
The mass of a substance remains constant. Here the ten vertical feet of peat material consisted of only one vertical foot of substantial coal/carbon deposits the remaining nine vertical feet of peat material had water which drained out during compaction through the course of time.
Thus except mass, all three i.e volume, density and area of a ten vertical feet of peat deposits must have changed when the said quantity reduced to one vertical foot of peat.
Thus, option C is correct.