An acidic buffer is obtained on mixing:

Acidic buffer = weak acid + its conjugate base (salt), with pH < 7.

Step 1 — Analyse each option:

(1) 100 mL 0.1M $\mathrm{CH_3COOH}$ + 100 mL 0.1M NaOH: Moles: acid = 0.01, base = 0.01 $\rightarrow$ complete neutralisation $\rightarrow$ only $\mathrm{CH_3COONa}$ (basic salt solution, not a buffer) ✗

(2) 100 mL 0.1M HCl + 200 mL 0.1M NaCl: HCl is strong acid, NaCl is neutral salt $\rightarrow$ not a buffer ✗

(3) 100 mL 0.1M $\mathrm{CH_3COOH}$ + 200 mL 0.1M NaOH: Moles: acid = 0.01, base = 0.02 $\rightarrow$ NaOH in excess $\rightarrow$ basic solution ✗

(4) 100 mL 0.1M HCl + 200 mL 0.1M $\mathrm{CH_3COONa}$: Moles: HCl = 0.01, $\mathrm{CH_3COONa}$ = 0.02 HCl reacts with $\mathrm{CH_3COONa}$: $\mathrm{HCl + CH_3COONa \rightarrow CH_3COOH + NaCl}$ After reaction: $\mathrm{CH_3COOH}$ = 0.01 mol, $\mathrm{CH_3COONa}$ = 0.01 mol $\rightarrow$ acidic buffer ✓

Answer: Option (4)