Explanation:
The correct answer is: B. Cl₂ is reduced because its oxidation number has decreased

Let’s break down the redox reaction:

\[
2Fe^{2+} + Cl_2 \rightarrow 2Fe^{3+} + 2Cl^-
\]

Step-by-step oxidation number analysis:

- Fe²⁺ → Fe³⁺:
- Iron goes from +2 to +3, meaning it loses one electron → oxidation.

- Cl₂ → 2Cl⁻:
- Chlorine in Cl₂ is 0 (elemental form), and in Cl⁻ it becomes -1.
- This is a gain of electrons, so chlorine is reduced.

Why is B correct?

- Reduction is defined as a decrease in oxidation number due to gain of electrons.
- Since Cl₂ goes from 0 to -1, its oxidation number decreases, making statement B accurate.

Let’s rule out the other options:

- A. Cl₂ is reduced because it has lost electrons — losing electrons is oxidation, not reduction.
- C. Cl₂ is reduced because its molecule is changed to two ions — this describes a change, but not the reason for calling it reduction.
- D. Fe²⁺ is reduced because it has lost electrons — losing electrons = oxidation.
- E. Fe²⁺ is reduced because it has gained electrons — Fe²⁺ lost electrons, so it was oxidized, not reduced.