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c. 0.2 M HNO₃ and 0.4 M NaF .

Explanation:

A buffer is defined as the mixture of a weak acid with its conjugate base or a weak base with its conjugate acid.

A weak acid or weak base are defined as an acid or base that partially dissociates in aqueous solution. in contrast, a strong acid or base are acids or bases that is dissociated completely in water.

Thus:

a. 0,2M HNO₃ and 0.4 M NaNO₃. This is a mixture of a strong acid with its conjugate base. IS NOT a buffer.

b. 0.2 M HNO₃ and 0.4 M HF . This is a mixture of two strong acids. IS NOT a buffer.

c. 0.2 M HNO₃ and 0.4 M NaF . NaF is the conjugate base of a weak acid as HF is.

The reaction of HNO₃ with NaF is:

HNO₃ + NaF → HF + NaNO₃

That means that in solution you will have a weak acid (HF) with its conjugate base (NaF). Thus, this mixture IS a buffer.

d. 0.2 M HNO₃ and 0.4 M NaOH. This is the mixture of a strong acid with a strong base, thus, this IS NOT a buffer.

I hope it helps!