Explain why it is not a good idea to throw an aerosol can into fire. Which gas law applies?

This is because the pressure will build so up so much that the can will burst (and explode if there is a naked flame nearby) the gas pressure increases. more of the liquefied propellant turns into a gas.

This is Boyle's Law. It states pressure and volume have an inverse relationship.

The gas inside the can and the can's volume are both constant.

The gas pressure increases with increasing temperature.

The can will burst if the pressure becomes great enough.

The gas law that applies is Gay-Lussac's law.