Flammability is a chemical property because combustion is a chemical change.
A chemical change is a process in which one or more substances are converted into one or more new substances with different properties.
Flammability is a measure of how easily something will burn.
Gasoline (petrol) is highly flammable. When it burns, it is converted into carbon dioxide and water. These have different properties from gasoline, so combustion is a chemical process.