Gabriel phthalimide synthesis is used for the preparation of

The correct option is primary amine.

Explanation:
Gabriel phthalimide synthesis is a classic organic reaction used specifically for the preparation of pure primary aliphatic amines (${\mathrm{R-NH}}_2$). This method avoids the formation of secondary or tertiary amines, which are common by-products in direct alkylation of ammonia.

The reaction proceeds through the following step-by-step mechanism:

1. Formation of Phthalimide Anion:
Phthalimide is treated with potassium hydroxide ($\mathrm{KOH}$). The strong base deprotonates the acidic imide nitrogen of phthalimide to produce potassium phthalimide, which contains a nucleophilic nitrogen anion.

2. Alkylation (Nucleophilic Substitution):
Potassium phthalimide is reacted with a primary alkyl halide ($\mathrm{R-X}$). The nucleophilic nitrogen attacks the alkyl group via an $S_{N}2$ mechanism, displacing the halide ion to yield N-alkylphthalimide.

3. Hydrolysis or Hydrazinolysis:
Finally, the N-alkylphthalimide is cleaved. This is typically achieved using hydrazine ($N_2{H}_4$) in a process called hydrazinolysis, or through acidic or basic hydrolysis. This cleavage releases the clean, uncontaminated primary amine along with phthalhydrazide as a side product.

Since the nitrogen atom in the phthalimide intermediate is bonded to two carbonyl groups, it can only undergo mono-alkylation. This structural feature prevents further alkylation steps, ensuring that the final product is strictly a primary amine rather than a secondary or tertiary amine.