The mixture that will produce a buffer solution when mixed in equal volumes is:

1. 0.1 mol dm-3 NH4OH and 0.1 mol dm-3 HCl
2. 0.05 mol dm-3 NH4OH and 0.1 mol dm-3 HCl
3. 0.1 mol dm-3 NH4OH and 0.05 mol dm-3 HCl
4. 0.1 mol dm-3 CH3COONa and 0.1 mol dm-3 NaOH