Which of the following illustrates the conservation of matter?
A. SnO2 + H2 --> Sn + 2 H2O
B. 2 C + O2 --> CO2
C. KOH + HBr --> KBr + H2O
D. 6 Cs + N2 --> 6 Cs3N

I want to say B?

C ) KOH + HBr → KBr + H₂O

Explanation:

Law of conservation of mass:

According to the law of conservation mass, mass can neither be created nor destroyed in a chemical equation.

This law was given by french chemist  Antoine Lavoisier in 1789. According to this law mass of reactant and mass of product must be equal, because masses are not created or destroyed in a chemical reaction.

For example:

C ) KOH + HBr → KBr + H₂O

in this equation there are equal number of atoms of each elements in both side so it means mass remain conserved.

While in other equations,

A) SnO₂ + H₂  →  Sn + 2H₂O

There are two hydrogen atoms on left side while four on the right side so mass is not conserved.

B) 2C + O₂    →   CO₂

There are two carbon atom on left side while one on right side. So this is also incorrect option.

C) 6Cs  + N₂  →    6Cs₃N

There are six Cs atoms on left side while 18 on right side. So this is also incorrect option.

i'm pretty sure its b i might be wrong though!