What is the purpose of a "level shifter" when interfacing two ICs operating at different voltage levels (e.g., 3.3V and 5V)?

A It converts signal voltage levels between two devices to prevent damage or unreliable communication caused by voltage mismatches B A level shifter permanently merges the two voltage levels into one C It is used only to increase the physical size of a circuit board D Level shifters convert digital signals into analog audio