Which of the following given reactions are examples of redox (oxidation reduction) reactions in basic inorganic chemistry?

Introduction / Context:
Redox reactions involve simultaneous oxidation and reduction, where one species loses electrons and another gains them. This question presents three common metal reactions and asks which of them are redox reactions. Recognising changes in oxidation state is the key skill being tested here.

Given Data / Assumptions:
- Three reactions are presented: combination with halogens and a displacement reaction with silver ions.
- We assume standard oxidation number rules for elements and ions.
- A redox reaction must show both oxidation and reduction in the same process.

Concept / Approach:
In a redox reaction, oxidation is loss of electrons or increase in oxidation number, and reduction is gain of electrons or decrease in oxidation number. Any reaction in which a metal in the elemental state reacts with a non metal or with metal ions usually involves the metal being oxidised and the partner species being reduced. Checking oxidation states before and after the reaction quickly reveals whether electron transfer occurs.

Step-by-Step Solution:
Step 1: For Ba(s) + Cl2(g) → BaCl2(s), assign oxidation numbers. Ba goes from 0 to +2 and each Cl goes from 0 to −1, so Ba is oxidised and Cl2 is reduced. Step 2: For Mg(s) + Br2(l) → MgBr2(s), Mg goes from 0 to +2 and each Br goes from 0 to −1, again showing oxidation of Mg and reduction of Br2. Step 3: For Al(s) + 3Ag+(aq) → Al3+(aq) + 3Ag(s), Al goes from 0 to +3 (oxidation) and Ag+ goes from +1 to 0 (reduction). Step 4: Recognise that all three reactions involve electron transfer and changes in oxidation states. Step 5: Conclude that all the listed reactions are redox reactions, so the correct choice is that all of the above are redox reactions.

Verification / Alternative check:
You can write half reactions for each process. For example, in the first reaction: Ba → Ba2+ + 2e− (oxidation) and Cl2 + 2e− → 2Cl− (reduction). Similar half reactions can be written for the magnesium and aluminium reactions. The presence of clear oxidation and reduction half reactions confirms that each overall reaction is redox in nature. No reaction listed is simply acid base or precipitation without electron transfer.

Why Other Options Are Wrong:
- Ba(s) + Cl2(g) → BaCl2(s) alone: This is a redox reaction but not the only one among the options, so choosing only this is incomplete.
- Mg(s) + Br2(l) → MgBr2(s) alone: Also a redox reaction, but again it is not unique among the three.
- Al(s) + 3Ag+(aq) → Al3+(aq) + 3Ag(s) alone: This displacement reaction is redox, but the question asks which of the following reactions are redox, not which single reaction is redox.
- None of these reactions are redox: This is clearly incorrect because each reaction shows changes in oxidation numbers.

Common Pitfalls:
A common mistake is to focus only on reactions involving oxygen and miss redox patterns elsewhere. Another pitfall is failing to assign oxidation numbers correctly to elements in their standard states, which always have oxidation number zero. Remember that any reaction involving a neutral metal forming ions or neutral non metals forming ions usually involves redox changes.

Final Answer:
Each of the listed reactions is a redox process, so the correct choice is All of the above reactions are redox reactions.