Correct Option (B)
The passage explicitly states that as the layers of sand and silt covering the dead marine organisms harden, they become sedimentary rock. This rock formation "effectively shut out the oxygen, so preventing the complete decomposition of the marine deposits underneath." Therefore, the exclusion of oxygen by the overlying rock layers is the direct reason for the incomplete decomposition of the mineral oil deposits.
Incorrect Options:
The passage mentions that ocean currents cover the dead creatures with sand and silt. However, it does not attribute the prevention of decomposition to the constant washing by these currents. The critical factor identified is the subsequent hardening of these layers into sedimentary rock.
The passage describes crude oil as a complex mixture of hydrogen and carbon. This refers to the chemical composition of the resulting crude oil, not the mechanism that prevents the complete decomposition of the original organic deposits.
While the organisms live in the sea, the passage does not identify saline conditions as the reason for the incomplete decomposition of their carcasses. The primary reason provided is the exclusion of oxygen due to sedimentary rock formation.