Which statement is true in the following reaction?
In the following reaction,
 $ \text{M}{{\text{g}}_{(\text{s})}}+\text{C}{{\text{u}}^{2+}}_{\text{(aq) }}\to \text{M}{{\text{g}}^{2+}}_{\text{(aq) }}+\text{C}{{\text{u}}_{(\text{s})}} $
1) $ \mathrm{Mg} $ is the reducing agent and $ \mathrm{Cu} $ is the oxidizing agent.
2) $ \mathrm{Mg}^{2+} $ is the reducing agent and $ \mathrm{Cu} $ is the oxidizing agent.
3) Cu is the reducing agent and $ \mathrm{Mg} 2+ $ is the oxidizing agent.
4) $ \mathrm{Cu}^{2+} $ is the reducing agent and $ \mathrm{Mg} $ is the oxidizing agent.
5) $ \mathrm{Mg} $ is the reducing agent and $ \mathrm{Cu}^{2+} $ is the oxidizing agent.

Hint: Redox (reduction–oxidation) is a chemical process that involves changing the oxidation states of atoms. The actual or formal transfer of electrons between chemical species is defined by redox reactions, which usually include one species (the reducing agent) suffering oxidation (losing electrons) while another species (the oxidising agent) experiences reduction (gains electrons).

Complete answer:
The chemical species that loses an electron is said to have been oxidised, whereas the chemical species that gains an electron is said to have been reduced. To put it another way:
The loss of electrons or a rise in the oxidation state of an atom, an ion, or specific atoms in a molecule is referred to as oxidation.
The gain of electrons or a drop in the oxidation state of an atom, an ion, or specific atoms in a molecule is referred to as reduction (a reduction in oxidation state).
Similar to acid–base reactions, oxidation and reduction occur concurrently and cannot occur independently of one another. Because two half-reactions usually occur together to produce a full reaction, oxidation and reduction are each called a half-reaction. In order to balance the half-reaction in terms of electric charge, the acquired or lost electrons are usually explicitly mentioned when writing half-reactions. When the half-reactions are combined to form the net chemical equation, the electrons cancel out.
In $ \text{M}{{\text{g}}_{(\text{s})}}+\text{C}{{\text{u}}^{2+}}_{\text{(aq) }}\to \text{M}{{\text{g}}^{2+}}_{\text{(aq) }}+\text{C}{{\text{u}}_{(\text{s})}} $
 $ \text{M}{{\text{g}}_{\left( \text{s} \right)}} $ acts as a reducing agent, contributing two electrons to the $ \mathrm{Cu}^{2+} $ ion while losing two electrons (oxidized).
 $ \mathrm{Cu}^{2+} $ acts as an oxidising agent, taking two electrons from the $ \text{M}{{\text{g}}_{\left( \text{s} \right)}} $ and reducing them in the process.
In the process of reducing a species, reducing agents (also known as reductants) are oxidised. By convention, oxidising substances (oxidants) are decreased.
These agents can also be located on the side of the reactants.
Hence option 5 is correct.

Note:
Reductive or reducing chemicals are known as reducing agents, reductants, or reducers because they have the capacity to reduce other compounds (causing them to acquire electrons). The reductant (reducing agent) is oxidised when it transfers electrons to another material. The reducing agent is also known as an electron donor since it donates electrons. Electron donors and electron acceptors can create charge transfer complexes.