Chelating agents can be used to remove toxic heavy metals from the human body by forming stable complex ions with the metal. Identify a ligand or ligand that is capable of removing excess lead. So remember we learned earlier that bidentate and polydentate ligands, or ligands are the best type of ligands to use as chelating agents.
If we take a look here, C is out because C is monodentated because only sulfa here can donate its lone pairs at that time. If we take a look at the others, we're going to say here that this oxygen is negative and this oxygen here is negative. But we can't count both of them as helping to be bidentated because they have to be separated by two or more atoms from one another. They're only separated by 1 carbon, so only one of them would actually act as a donor atom at a time. So this is out.
A is the answer because an A we have this oxygen single bonded, so its charge is -1. Same with this sulfur and same with this oxygen and same with this sulfur. Each of these negatively charged or -1 formal charged atoms is at least two atoms away from the next one, so they each connect as a donor atom. In this case we have 4 donor atoms within this structure so it'd be polydentate so it could form or act as a chelating agent.
So out of all the options, only option A is the correct choice.