On a hydrocarbon flammability chart the line which extends from 0% to 21.8% oxygen, lying tangent to the flammability range, is called the __________.

Correct Answer: Option B — critical dilution line

The critical dilution line on a flammability diagram represents the boundary where any mixture of hydrocarbon gas and air, when diluted with air, will not enter the flammable range. It is a vital reference line for safe purging and gas-freeing operations on tankers.