Buffers are solutions that resist changes in pH when small amounts of acid or base are added to them. They consist of a weak acid and its conjugate base or a weak base and its conjugate acid. The key components of a buffer system are the weak acid-base pair that can donate or accept protons to maintain the pH of the solution.

Let's analyze the given options to determine which can act as a buffer:

1) NH4OH (ammonium hydroxide) and NaOH (sodium hydroxide):
- NH4OH is a weak base, and NaOH is a strong base.
- Since NH4OH is a weak base, it can act as a buffer when combined with its conjugate acid, NH4+.
- However, NaOH is a strong base and does not have a conjugate acid to form a buffer system.
- Therefore, this option cannot act as a buffer.

2) HCOOH (formic acid) and CH3COONa (sodium acetate):
- HCOOH is a weak acid, and CH3COONa is the salt of a weak acid (acetic acid).
- Both HCOOH and CH3COONa can form a buffer system.
- HCOOH can donate protons, and CH3COO- can accept protons, maintaining the pH of the solution.
- Therefore, this option can act as a buffer.

3) 40ml 0.1M NaCN and 20ml of 0.1 M HCl:
- NaCN is the salt of a weak acid (hydrocyanic acid).
- HCl is a strong acid.
- The addition of a strong acid (HCl) to the salt of a weak acid (NaCN) can disrupt the buffer system.
- The strong acid will protonate the weak base (CN-) and shift the equilibrium towards the formation of the weak acid (HCN).
- Therefore, this option cannot act as a buffer.

4) None of them:
- This option is correct since only option 2 (HCOOH and CH3COONa) can act as a buffer.

In conclusion, option 2 (HCOOH and CH3COONa) can act as a buffer system because it consists of a weak acid (HCOOH) and its conjugate base (CH3COO-). Option 3 (40ml 0.1M NaCN and 20ml of 0.1 M HCl) cannot act as a buffer because the addition of a strong acid disrupts the buffer system.