You need to apply the definition. I assume you are using the Bronsted-Lowry theory. Acids donate a proton; bases accept a proton.
H2C2O4, for example, can ONLY donate a proton (you know there is no such thing as H3C2O4 or H4C2O4) so it looks like this. H2C2O4 ==> H^+ + HC2O4^-. Another for example is that HC2O4^- can act as both an acid and a base. WHY? Because it can donate a H^+ as in HC2O4^- ==> H^+ + C2O4^2- (the fact that H^+ and C2O4^2- are listed in the same question should tell you that those exist so it can go to the right and act as an acid OR it can do HC2O4^- + H^+ ==> H2C2O4 and act as a base.) That takes care of two of them; I'll leave the other two for you.
An oxalic acid solution contains the following species in varying concentrations: H2C2O4, HC2O4-, C2O4^2-, and H+. Which of the above four species can act only as acids, which can act only as bases, and which can act as both acids and bases?
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