Answer:
The addition of NH₃⁺
Explanation:
A buffer is an aqueous mixture of a weak acid and its conjugate base or vice versa.
In ammonium iodide, NH₄I, the ions in solution are ammonium ion, NH₄⁺ and iodide ion, I⁻.
The iodide ion is the conjugate base of the HI, a STRONG ACID, that means the addition of HI in solution wouldn't produce a buffer.
And, NH₄⁺ is the conjugate acid of the NH₃, a WEAK BASE, that means the addition of NH₃ to the solution will produce a buffer: The mixture of a weak base, NH₃ and its conjugate acid, NH₄⁺
