Generally, oxygen has an oxidation number of , and hydrogen has an oxidation number of . If we multiply  by the number of oxygen atoms in the compound (), we get an overall charge of  on the oxygen. If we multiply  by the number of hydrogen atoms in the compound (), we get an overall charge of  on the hydrogen. The sum of  and  is . We can see that  is a neutral molecule, so the charge on  would have to balance out the  charge on the rest of the molecule. To obtain a charge of , the oxidation number on  must then be .

The answer is .

Lindlar's catalyst enables catalytic hydrogenation, specifically for an alkyne to reduce to a cis-alkene (though the reaction does not continue to produce an alkane). The rest of the answer options are oxidizing agents.

Water, , has an oxygen atom with an oxidation number of  whereas hydrogen peroxide, , has an oxygen atom with an oxidation number of . A less negative oxidation number suggests that the oxygen lost an electron. Recall that oxidation is the loss of electrons whereas reduction is the gain of electrons; therefore, the oxygen atom lost an electron and was oxidized when water was converted to hydrogen peroxide.

Oxidation is the process of removing electrons from an atom. This increases the oxidation number (makes oxidation number more positive). An atom undergoes oxidation by losing electrons and donating them to another atom. Since it is facilitating reduction (gain of electrons) of another atom, an atom that is oxidized is also called a reducing agent. Oxidizing agents, on the other hand, are reduced (gain electrons) and facilitate the oxidation of other atoms (removal of electrons from other atoms). Oxidation of an atom does not depend on the Gibbs free energy; therefore, oxidation can be spontaneous or nonspontaneous.

 is the only option that will not reduce a ketone to an alcohol, simply because it is not a reducing agent like the other four—it is an oxidizing agent. The addition of oxygens to a ketone will not yield an alcohol (oxidation). The addition of hydrogen to a ketone will yield an alcohol (reduction). The other answer options are reducing agents that would facilitate this reaction.

Reduction involves gain of electrons whereas oxidation involves loss of electrons. In reduction, the oxidation number becomes more negative (due to gain of electrons) whereas in oxidation, the oxidation number becomes more positive. Sodium is an alkali metal, found on the first column of the periodic table. Every atom in this column has one valence electron; therefore, to complete its octet every alkali metal will lose an electron and will have an oxidation number of . It is very hard to oxidize and reduce these metals because of its desire to maintain octet; therefore, sodium can never be reduced or oxidized (it will always have an oxidation number of ).

Oxidation and reduction involve loss and gain of electrons, respectively. It does not involve loss or gain of protons. Recall that the identity of an atom is changed when the amount of protons change; therefore, it is very hard to change the amount of protons. Only few reactions (such as nuclear decay reactions) can change the number of protons and alter the identity of an atom.