Which has the maximum number of molecules among the following?

Which of the following is incorrect with respect to $Chondrichthyes$?

The binding energy of deuteron ${}_1^2H$ is $1.112 \text{ MeV}$ per nucleon and an $\alpha$-particle ${}_2^4He$ has a binding energy of $7.047 \text{ MeV}$ per nucleon. Then in the fusion reaction ${}_1^2H + {}_1^2H \to {}_2^4He + Q$,the energy $Q$ released is ........... $\text{MeV}$.

The Phthalein test is a characteristic test for which of the following?

The vapour pressure of a solvent decreases by $10 \ mm$ of mercury when a non-volatile solute is added to the solvent. The mole fraction of the solute in the solution is $0.2$. What should be the mole fraction of the solute if the decrease in the vapour pressure is to be $20 \ mm$ of mercury?

In a biprism experiment,if the $5^{\text{th}}$ bright band with wavelength $\lambda_{1}$ coincides with the $6^{\text{th}}$ dark band with wavelength $\lambda_{2}$,then the ratio $\left(\frac{\lambda_{2}}{\lambda_{1}}\right)$ is: