Sublimation temperature of dry ice (solid CO2) is __________ °C.

Answer: Option C

Sublimation means the conversion of solid phase directly in to vapor phase (without involvement of phase liquid). Actually for every substance this direct phase conversion occurs when the substance is below its triple point pressure and corresponding temperature.
Actually one of the point where the dry ice sublimates is 1 atm pressure and corresponding temperature is $$-78^{\circ }C.$$