A. 0.01 °C (Correct)
Triple point of a substance is defined as a point of temperature & pressure in which the substance can exist in all it’s three states of solid, liquid & gas.
The single combination of pressure and temperature at which liquid water, solid ice, and water vapor can coexist in a stable equilibrium occurs at exactly 273.16K (0.01 °C) 32.08º F and a partial vapor pressure of 611.73 pascals(ca. 6.1173 millibars, 0.0060373 atm). At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature.