Answer
a) Not a Buffer.
b) Is a Buffer.
c) Not a Buffer.
d) Not a Buffer
Work Step by Step
a) Hydrobromic acid is a strong acid and NaCl is not a weak conjugate pair of $HBr$.
b) $HCHO_{2}$ and $NaCHO_{2}$ are a buffer because $HCHO_{2}$ is a weak acid and $NaCHO_{2}$ is the conjugate pair of that weak acid.
c) $HCl $and $HBr$ are both strong acids and thus cannot produce a buffer as you need one species of an acid and one of a base that must be a conjugate pair.
d)$KOH$ and $NH_{3}$ are both bases and thus can't form a buffer as you need one species of an acid and one of a base that must be a conjugate pair.
