1. Basicity in aqueous solution depends on two main factors:

   • $+I$ effect of alkyl groups, which increases electron density on nitrogen and tends to increase basicity.
   • Solvation of the conjugate acid in water. Better solvation stabilizes the protonated amine and tends to increase basicity.

2. Compare ammonia and hydrazine:

   • In hydrazine, $\mathrm{H_2N-NH_2}$, one $-\mathrm{NH_2}$ group exerts a $-I$ effect on the other nitrogen.
   • This reduces the availability of the lone pair compared with $\mathrm{NH_3}$.
   • Therefore, $\mathrm{NH_2NH_2 < NH_3}$

3. Compare alkyl amines in aqueous solution: For aliphatic amines in water, the general order is: $\text{secondary amine} > \text{primary amine} > \text{tertiary amine} > \mathrm{NH_3}$ This is because:

   • Secondary amines have strong $+I$ effect and still good solvation.
   • Primary amines are next.
   • Tertiary amines, though electronically stronger, are less solvated due to steric hindrance, so in aqueous solution they are less basic than primary and secondary amines.

   Hence for ethylamines: $\left(\mathrm{CH_3CH_2}\right)_2\mathrm{NH} > \mathrm{CH_3CH_2NH_2} > \left(\mathrm{CH_3CH_2}\right)_3\mathrm{N}$

4. Combine all species: From steps 2 and 3, $\mathrm{NH_2NH_2 < NH_3 < \left(CH_3CH_2\right)_3N < CH_3CH_2NH_2 < \left(CH_3CH_2\right)_2NH}$

5. Match with options: This corresponds to Option B.

6. Check against stored answer:

   • Stored answer: C
   • Derived answer: B

   Option C places triethylamine more basic than ethylamine: $\mathrm{CH_3CH_2NH_2 < \left(CH_3CH_2\right)_3N}$ which is not correct in aqueous solution because tertiary amines are less solvated than primary amines.

Therefore, the stored answer appears incorrect.