1. Check option A: Compare intermolecular association in primary vs secondary amines.

Primary amines have the group $RNH_2$ and secondary amines have $R_2NH$.

• In primary amines, there are two N–H bonds, so hydrogen bonding is more extensive.
• In secondary amines, there is only one N–H bond, so hydrogen bonding is less extensive.

Therefore, intermolecular association is greater in primary amines than in secondary amines.

So the statement $\text{“The intermolecular association in primary amines is less than that in secondary amines”}$ is incorrect.

2. Check option B: Reaction with nitrous acid.

Primary aliphatic amines react with nitrous acid to form unstable diazonium salts, which decompose to give alcohols: $RNH_2 + HNO_2 \rightarrow ROH + N_2 + H_2O$

Example: $C_2H_5NH_2 \rightarrow C_2H_5OH$

For methyl amine: $CH_3NH_2 + HNO_2 \rightarrow CH_3OH + N_2 + H_2O$

So methyl amine also gives the corresponding alcohol, methanol. Hence the phrase "except methyl amine" makes the statement incorrect.

So option B is also incorrect as written.

3. Check option C: Basic strength.

For aliphatic amines in aqueous solution, the general order is: $2^\circ > 1^\circ > 3^\circ > NH_3$

Thus primary amines are less basic than secondary amines. So C is correct.

4. Check option D: Gabriel phthalimide synthesis.

Gabriel synthesis is used to prepare primary aliphatic amines: $\text{Phthalimide} \xrightarrow[\text{alkyl halide}]{} \text{N-alkyl phthalimide} \xrightarrow[\text{hydrolysis}]{} RNH_2$

So D is correct.

5. Conclusion

• A is incorrect.
• B is also incorrect because methylamine does form methanol with nitrous acid.
• C and D are correct.

Since this is marked as an MCQ (single correct), the intended textbook answer is likely A, based on the standard comparison of hydrogen bonding in primary and secondary amines. However, strictly speaking, B is also not correct as written.