Which among the following pairs constitutes a buffer?

Calculate the $pH$ of a buffer solution prepared by mixing $30 \ mL$ of $0.1 \ M$ $NaOH$ and $100 \ mL$ of $0.1 \ M$ $CH_3COOH$. (Given: $pK_a$ of $CH_3COOH = 4.76$)

What is $[H^{+}]$ of a solution that is $0.01 \ M$ in $HCN$ and $0.02 \ M$ in $NaCN$ ($K_a$ for $HCN = 6.2 \times 10^{-10}$)?

$A$ buffer solution is a mixture of

What will be the change in $pH$ by adding $0.1 \ M \ NH_4Cl$ to $0.1 \ M \ NH_4OH$ (weak base) solution? (Given: $K_b$ of $NH_4OH = 1.77 \times 10^{-5}$)

$50 \, mL$ of $2 \, N$ acetic acid mixed with $10 \, mL$ of $1 \, N$ sodium acetate solution will have an approximate $pH$ of $(K_a = 10^{-5})$