Doubt from buffer

Answer is c,d
Why c?

according to definition strong acid is needed for buffer which is hno3..

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But how concentrations are same then in the example written with pencil?

I actually saw this on quora:

HNO3 is a strong acid. So I thought it will ionise completely.

But I am not sure if 10 mmoles of ch3coona will completely react with H+ :confused: