Answer: (b) HCl
The eggshell contains calcium carbonate, which on reaction with HCl liberates CO2 gas which turn lime water to milky.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
The gas thus formed is an effervescent carbon dioxide gas when calcium carbonate reacts with hydrochloric acid. The carbon dioxide gas also acts as an extinguisher for fires.