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Consider the conjugate acid-base pair HA and its equivalent base. Because they are linked to proton gain or loss, they are collectively known as conjugate acid-base pair.

A weak base is a conjugate base of a weak acid.

A weak acid is a conjugate acid of a weak base$\mathrm{pk}$

The negative logarithm of an equilibrium constant is called as pK

List of bases with their respective ${\mathrm{pK}}_{\mathrm{a}}$value

Compound ${\mathrm{pK}}_{\mathrm{a}}$(for conjugate acid)

Ammonia 9.24

Aniline $4.60$

Hydrazine $8.02$

Pyridine 5.20

From the above given list, it can be seen that Ammonia has ${\mathrm{pK}}_{\mathrm{a}}$value that is closest for the preparation of buffer at role="math" localid="1654765470337" $\mathrm{pH}=9.00$

Hence,

Ammonia can be used as suitable base for preparing buffer of $\mathrm{pH}=9.00$

Reason for incorrect option:

Aniline and other two bases can be toxic even though if they had role="math" localid="1654765611815" ${\mathrm{pK}}_{\mathrm{a}}$value near to the given pH

Hydrazine and Pyridine cannot be used as suitable buffer since their${\mathrm{pK}}_{\mathrm{a}}$ values aren't close to the that is required for the preparation of buffer.

Conclusion

Ammonia is found to be most suitable base, because it has ${\mathrm{pK}}_{\mathrm{a}}$ value that is closest for the preparation of buffer at$\mathrm{pH}=9.00$