Aluminium powder is used in thermit welding because:

The NCERT text (Section 3.4.4) states that the reaction of iron(III) oxide ($\text{Fe}_2\text{O}_3$) with aluminium powder is highly exothermic and is called the thermit reaction:

$$\text{Fe}_2\text{O}_3(s) + 2\text{Al}(s) \rightarrow 2\text{Fe}(l) + \text{Al}_2\text{O}_3(s) + \text{Heat}$$

The book explicitly says: 'The amount of heat evolved is so large that the metals are produced in the molten state. In fact, the reaction of iron(III) oxide with aluminium is used to join railway tracks or cracked machine parts.' So it is the molten iron produced by the reaction — poured into the gap between rail ends — that solidifies and welds the tracks together.

• (A) Incorrect — the reaction is indeed exothermic, but exothermicity alone does not explain the welding application; it is the resulting molten iron (not the heat release itself) that physically joins the tracks.
• (B) Correct — this directly matches the NCERT description: molten iron (Fe(l)) is obtained and used to weld the rails.
• (C) Incorrect — it wrongly claims molten aluminium oxide ($\text{Al}_2\text{O}_3$) is used to join the tracks; $\text{Al}_2\text{O}_3(s)$ is a solid by-product with a very high melting point (2323 K) and plays no role in welding — it is the molten iron that does.
• (D) Incorrect — this option fabricates chemistry not present in the NCERT text: the thermit reaction does not form 'a molten alloy of iron and aluminium'; the products are separate molten iron, Fe(l), and solid aluminium oxide, $\text{Al}_2\text{O}_3(s)$, as shown in the balanced equation. There is also no comparison of melting points made in the chapter to justify this option.