In order for an ionic compound to be created, there has to be a reaction between a metal and a non-metal. Having this in mind, the correct answer is A. strontium and chlorine. Strontium (Sr) is an alkaline earth metal, whereas chlorine (Cl) is a halogen gas, so a non-metal. There is no this type of a combination of metal + non-metal in other options, so only A is correct.
