The order of basicity decreases with decrease in alkylation in the gas phase. This trend is changed in aqueous medium. The appropriate reason for this change is (a) the electron releasing effect of alkyl group. (b) difference in behaviour of alkyl group in aqueous phase. (c) ammonium ion formed in tertiary amine does not have effective hydrogen bonding. (d) alkyl group stabilises only the alkyl ammonium ion.

Basicity is a measure of how easily a compound can act as a base by accepting a proton (H+ ion) to form a conjugate acid. Factors affecting basicity include the availability of the lone pair of electrons (for proton acceptance) and the stability of the conjugate acid formed after accepting the proton.

Let's analyze each of the given options: (a) The electron-releasing effect of the alkyl group: This would indeed increase the basicity of amines as alkyl groups donate electrons to the nitrogen atom, making it more available to accept a proton. However, this does not explain the change in trend when moving from the gas phase to the aqueous medium. (b) Difference in behavior of alkyl group in aqueous phase: This option is vague and does not provide a specific reason for the change in trend. (c) Ammonium ion formed in tertiary amine does not have effective hydrogen bonding: In the aqueous phase, hydrogen bonding plays a significant role in stabilizing the conjugate acids formed upon accepting a proton. Amines with more effective hydrogen bonding in the aqueous phase will have higher basicity. (d) Alkyl group stabilizes only the alkyl ammonium ion: This statement does not offer a clear explanation for the change in trend when moving from the gas phase to the aqueous medium.

From the analysis in Step 2, the most plausible explanation for the change in the order of basicity with the decrease in alkylation from the gas phase to the aqueous medium is option (c), which highlights the importance of hydrogen bonding in the aqueous phase. Tertiary amines lack a direct N-H bond required for strong hydrogen bonding with water molecules, so their ammonium ions are not as well-stabilized as primary or secondary amine ammonium ions. Hence, the correct answer is (c) ammonium ion formed in tertiary amine does not have effective hydrogen bonding.