The most commonly used reducing agent is ------------

Explanation

`+4` oxidation state of Sn is more stable than `+2` oxidation state. Therefore `Sn^(2+)` can be easily oxidised to `Sn^(4+)` and hence `SnCl_(2)` acts an reducing agent.
`SnCl_(2) + 2Cl^(-) rarr SnCl_(4) + 2e^(-)`