The answer should be A, flammability.
The isotopes of the same element would result in the same chemical properties, as the number of protons and electrons are not changed, meanwhile, the physical properties would be slightly different with each other because they have different numbers of electrons.
Out of the 4 options, only option A is a chemical property. Chemical properties are the properties of an element reacting with other substances that creates new substances at the end. While physical properties are the ones that does not produces new substances or any reactions. They are usually observable and measurable.
Therefore, since flammability is a chemical property, the answer is A.
