In aniline, acetanilide and p-nitroaniline, there is a delocalisation of the lone pair of nitrogen in the ring due to resonance and due to the presence of electron withdrawing groups which decrease the electronic charge of amino group. But in benzyl aniline, there's no delocalisation of electrons of nitrogen in the ring and also absence of EWG which makes it most basic among these. So, option a is correct.

Explanation:

Benzylamine:
- Benzylamine is the most basic compound among the options provided.
- It is a primary aliphatic amine with the formula C₆H₅CH₂NH₂.
- The presence of the amino group (-NH₂) makes benzylamine basic.
- In a basic compound, the amine group can accept a proton (H⁺) to form an ammonium ion.
- Benzylamine has a higher basicity compared to aromatic amines like aniline and its derivatives due to the presence of an alkyl group which stabilizes the ammonium ion formed after accepting a proton.
- It is a simple organic compound and lacks any substituents that can influence its basicity.

Aniline:
- Aniline is less basic compared to benzylamine because of the presence of the aromatic ring.
- The delocalization of electrons in the aromatic ring decreases the availability of the lone pair on the nitrogen atom for protonation.
- Aniline is a primary aromatic amine with the formula C₆H₅NH₂.

Acetanilide:
- Acetanilide is an amide derivative of aniline and is less basic than aniline.
- The presence of the acetyl group (-NHCOCH₃) reduces the basicity of acetanilide compared to aniline.

p-Nitroaniline:
- p-Nitroaniline is a derivative of aniline with a nitro group (-NO₂) attached to the benzene ring.
- The presence of the electron-withdrawing nitro group further decreases the basicity of p-nitroaniline compared to aniline.

Therefore, among the given options, benzylamine is the most basic compound due to its simple structure without any substituents that can hinder its basic nature.