To understand why option 'D' is the correct answer, let's first discuss the direct nitration of aniline and the formation of meta derivative.

1. Direct Nitration of Aniline:
Aniline is a primary aromatic amine that contains a lone pair of electrons on the nitrogen atom. When aniline is treated with a nitration reagent, such as nitric acid (HNO3) and sulfuric acid (H2SO4), a nitro group (-NO2) is introduced onto the benzene ring through an electrophilic aromatic substitution reaction.

2. Formation of Meta Derivative:
During the nitration reaction, the nitronium ion (NO2+) acts as the electrophile and attacks the benzene ring of aniline. However, due to the presence of the amino group (-NH2) on the ring, the electron density is higher at the ortho and para positions compared to the meta position.

This higher electron density at the ortho and para positions makes them more susceptible to electrophilic attack, resulting in the formation of ortho and para derivatives. On the other hand, the meta position, being less electron-rich, is less reactive and leads to the formation of the meta derivative.

Now, let's discuss why each of the options can help obtain more p-nitro derivative.

a) Reacting with Acetic Anhydride:
Acetic anhydride (CH3CO)2O can be used to acetylate aniline, converting it into N-acetylaniline. This acetylation reaction blocks the amino group (-NH2) of aniline, reducing its electron-donating ability. As a result, the electron density on the benzene ring decreases, making the meta position less deactivated and more susceptible to electrophilic attack by the nitronium ion. This increases the chances of obtaining the p-nitro derivative.

b) Acetylation Reaction:
As mentioned above, the acetylation reaction with acetic anhydride reduces the electron-donating ability of aniline. This decreases the electron density on the benzene ring, making the meta position less deactivated and favoring the formation of the p-nitro derivative.

c) Controlling the Nitration Reaction:
By carefully controlling the reaction conditions, such as temperature, concentration, and reaction time, it is possible to favor the formation of the p-nitro derivative. Lowering the temperature and using a lower concentration of the nitration reagents can help slow down the reaction and increase the chances of nitration occurring at the meta position.

d) All of these:
By combining the acetylation reaction with acetic anhydride and controlling the nitration reaction conditions, we can increase the chances of obtaining the p-nitro derivative. Acetylation reduces the electron-donating ability of aniline, while controlling the nitration reaction favors nitration at the meta position. Together, these techniques can enhance the selectivity towards the p-nitro derivative.

In conclusion, option 'D' is the correct answer because all of the mentioned techniques (reacting with acetic anhydride, acetylation reaction, and controlling the nitration reaction) can be employed to obtain a higher yield of the p-nitro derivative by reducing the reactivity of the meta position.