Identify the molecule / ion in which the ratio of $\sigma$ to $\pi$-bonds is $3: 2$

At 298 K , a flask ' $A$ ' of unknown volume $(V)$ contains oxygen at 5 atm . Another flask ' $B$ ' of volume 2 L contains helium at 3 atm . Two flasks are connected together by a small tube of zero volume. After the two gases are completely mixed, if the resulting mixture is found to have the mole fraction of oxygen as 0.2 , the volume of flask ' $A$ ' (in L ) is

(Assume oxygen and helium as ideal gases)

In which of the following, oxidation state of nitrogen is lowest?

Which of the following processes are reversible?

I. Vaporisation of a liquid at its boiling point.

II. Expansion of gas into vacuum.

III. Transformation of a solid substance into liquid at its melting point.