Bottle of ${\text{PC}}{{\text{l}}_{\text{3}}}$ is kept stoppered because it:
A. explodes
B. gets oxidised
C. it volatilized
D. reacts with moisture

Hint:Phosphorous trichloride is a colourless, and fuming liquid at room temperature. HCl gas is produced by the reaction of phosphorus trichloride with water. The oxidation state of phosphorus in ${\text{PC}}{{\text{l}}_{\text{3}}}$ is $ + 3$. It undergoes oxidation to form P(V).

Complete answer:
Phosphorous trichloride undergoes oxidation reaction to form Phosphorous pentachloride but the reaction does not occur simply in presence of moisture. A proper chemical reaction and condition are required for the oxidation.
When a substance vaporizes even at low temperature it is known as volatile. Generally, the substances that exist as a gas at room temperature are volatile. At room temperature, phosphorus trichloride exists as a liquid but it is volatile.
When phosphorus trichloride comes in contact with moisture it undergoes hydrolysis.it reacts violently with water. The reaction of phosphorus trichloride with water gives fumes of hydrogen chloride gas.
The reaction is as follows:
${\text{PC}}{{\text{l}}_{\text{3}}}\,{\text{ + }}\,3\,{{\text{H}}_{\text{2}}}{\text{O}}\, \to \,{{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}\,{\text{ + }}\,{\text{3}}\,{\text{HCl}}$
The phosphorous acid and hydrogen chloride both are corrosive.
So, it is true that phosphorus trichloride reacts with moisture and it is volatile and gets oxidized. But all these happen due to the reaction with water, so the main reason to keep it in a closed bottle is the reaction with moisture. The products of the reaction of phosphorus trichloride with water are harmful, corrosive chemicals.

Therefore, option (D) reacts with moisture is correct.

Note:
Phosphorus trichloride fumes in moisture due to the formation of hydrogen chloride gas. Phosphorus trichloride is toxic and corrosive. Its reaction with water is not so explosive but vigorous. Hybridization of phosphorus on ${\text{PC}}{{\text{l}}_{\text{3}}}$ is ${\text{s}}{{\text{p}}^{\text{3}}}$ . The geometry is trigonal pyramidal.