The complete combustion of which of the following substances produces carbon dioxide and water

C8 H18 is the answer

I found the whole exercise on the internet. These are the options:
"A) CaHCO3
B) C8H18
C) NO
D) K2CO3"

Correct  B) C8H18
This compound is called octane. In its combustion are envolved 2 molecules of C8H18 (octane) itself and 25 molecules of O2 (oxygen). This combustion/reaction leads to the formation of 16 molecules of CO2 (carbon dioxide) and 18 molecules of H2O (water). This is a compound that's quite popular due to its presence in gasoline.

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged