Answer:
c. CaI_2 calcium iodide.
Step-by-step explanation:
It is a common rule that vapor pressure is inversely proportional to boiling point of the substance. That is larger the boiling point smaller will be the vapor pressure.
B.P. of Ar= -185.8 °C
B.P. of CO2= -78.46 °C
B.P. of calcium iodide= 1,100 °C
B.P. of BF3 = -100.3 °C
B.P. of PF5= −84.6 °C
clearly, Boiling point of calcium iodide is highest hence, its vapor pressure is least among all the options.