Oxidation and reduction are defined in terms of electron transfer:
Oxidation is the loss (removal) of electrons from a species.
Reduction is the gain (addition) of electrons to a species.
A useful mnemonic is OIL RIG: Oxidation Is Loss, Reduction Is Gain.
For example, when iron is oxidised:
\[\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-\]
Iron loses two electrons, so its oxidation state increases from 0 to +2. The electrons are removed from the iron atom.
The other options are incorrect: "added" describes reduction (the opposite process), while "hydrolysed" (broken down by water) and "hydrated" (combined with water molecules) are unrelated to the electron-transfer definition of oxidation.
