Pure substances are substances made of only one compound, and mixtures, substances made of two or more compounds. To separate the compounds of a mixture, it can be used chemical or physical separation. For example, to separate sand and water it may be used as a filter, which is a physical separation. But if there are small particles in the water, maybe it would be necessary coagulation with some chemical element before a filtration, which would be a chemical separation.

To separate the elements of a pure substance, only chemical separations can be used: would be necessary a chemical reaction to form H2 and O2 from H20, for example. So, letter A is correct.

Letter B is incorrect because can be differents atoms in the mixture, depends on its components.

Letter C is incorrect because all molecules, compounds, and substances have chemical bonds, it’s the force that let the elements be together.

Letter D is incorrect because, as explained, pure substances only can be separated in their elements by a chemical reaction.