The most commonly used reducing agent among the following is :

1. AlCl3 

2. PbCl2

3. SnCl4

4. SnCl2

 
 



Hint: Element stable in higher oxidation state than lower oxidation state show strong reducing agent

Reducing agents are those substances that reduce other substances and itself oxidises.
 
In SnCl2, Sn oxidises in +2 oxidation state, thus, acts as a strong reducing agent i.e.,
 
SnCl2+2FeCl32FeCl2+SnCl4
SnCl2+2CuCl22CuCl+SnCl4