Compounds can be separated by

chemical means
physical means
boiling
pressure

In order to compounds to be serparated, chemical means are needed. For example, if it you want to separate hydrogen and oxygen in the water ,an electric current should flow through it and that is a kind of chemical means(it is related to the chemical properties of the elements .)
While ,if there is a mixture ,it's components are rather separated easily in physical means. For instance, if you have a salt water, you are able to divide them by using each physical properties. Water is able to turn to a state of gas by evaporating or boiling. Using such properties, you could separate water from salt. Therefore, separating compounds and mixtures are all about the components physical or chemical properties.

So, the answer is a. I hope it will help you^^