The decreasing order of basicity of following amines in aqueous phase…

The correct answer is Option (1) → (C), (B), (A), (D)

In aqueous solution, basicity of aliphatic amines depends on:

• +I effect of alkyl groups (electron donation increases basicity).
• Solvation of the ammonium ion (more hydrogen bonding $\rightarrow$ more stabilization).

Order in water:

$\text{Secondary amine} > \text{Primary amine} > \text{Tertiary amine} > \text{Aromatic/benzyl-type amine} \text{}$

Explanation of each Amine

(C) N-Ethylethanamine (diethylamine) – Secondary amine:

Has two alkyl groups giving strong +I effect, increasing electron density on nitrogen. Its conjugate acid ($R_2NH_2^+$) is still well solvated by water via hydrogen bonding. Hence, it shows the highest basicity among the given.

(B) Methanamine (methylamine) – Primary Amine

Has one alkyl group giving moderate $+I$ effect. Its ammonium ion ($RNH_3^+$) forms strong hydrogen bonds with water, giving good stabilization. Thus, it is less basic than secondary but more than tertiary amine.

(A) N,N-Dimethylmethanamine (trimethylamine) – Tertiary Amine

Has strong $+I$ effect from three alkyl groups, but its conjugate acid ($R_3NH^+$) is poorly solvated due to steric hindrance and fewer H-bonds. Reduced solvation lowers stability of the protonated form, decreasing basicity in water.

(D) Phenylmethanamine (benzylamine)

The benzyl group is attached to nitrogen through $-CH_2-$, but the aromatic ring exerts a $-I$ effect. Overall electron density on nitrogen is less compared to aliphatic amines. Thus, it is the least basic here.