Explain why a propane torch is lit inside a hot air balloon during preflight preparations. which gas law applies?

Propane torch is lit inside a hot air balloon during pre-flight preparation because the heat from the touch is needed to heat the cold air inside the balloon, so that the air will expand and become less dense and rise, thus providing a lift for the balloon. This is line with charle's law, which states that, the volume of a fixed mass of ideal gas is directly proportional to the absolute temperature. This law implies that, as the temperature of the air inside the balloon increase, the volume of the balloon also increases.

The propane torch is used to heat up the cold air present inside the balloon and this is because as the cold air gets warmer it starts to expand and it will also lose its density ( i.e. it becomes  less dense ) which leads to the rising of the balloon.

Charles's law states that provided the mass of an ideal gas is fixed, the temperature of the gas is directly proportional to its volume ( i.e. increase in temperature = increase in volume ).  This is why as the cold air gets warmer its' volume increases ( expands )

Hence we can conclude that Propane torch is lit inside a hot air balloon ;  To heat and expand the air inside the balloon. The gas law that is applicable is ;   Charles's Law

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answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.