1. Key idea: stabilization of low oxidation states

   Metals in low oxidation states are electron-rich. Such metals are stabilized by ligands that can accept electron density from the metal.

2. Role of $\pi$-acceptor ligands

   Ligands with good $\pi$-accepting character have vacant $\pi^*$ orbitals. They can accept electron density from filled metal $d$-orbitals through back-bonding:

   $M(d\pi) \to L(\pi^*)$

   This removes excess electron density from the metal and therefore stabilizes the metal in a low oxidation state.

   Examples: CO, CN$^-$, phosphines.

3. Examine each option

   A: have good $\pi$-accepting character
   This is correct, because $\pi$-acceptor ligands stabilize electron-rich, low oxidation state metals by back-bonding.

   B: have good $\sigma$-donor character
   Strong $\sigma$-donor ligands donate more electron density to the metal, which generally favors higher electron density on the metal and does not specifically stabilize low oxidation states as effectively as $\pi$-acceptors.

   C: are having good $\pi$-donating ability
   $\pi$-donor ligands donate electron density to the metal, which is more suitable for stabilizing metals in higher oxidation states, not lower ones.

   D: are having poor $\sigma$-donating ability
   Poor $\sigma$ donation does not provide the key stabilization needed. The important factor is acceptance of electron density by back-bonding, not weak donation.

4. Conclusion

   Therefore, low oxidation states of metals are common when ligands:

   $\boxed{\text{have good } \pi\text{-accepting character}}$

So the correct option is A.