Aluminium powder is used in thermite welding because:

The correct option is A

it is a strong reducing agent

Explanation for the correct option:

(A) it is a strong reducing agent

• Thermite welding is a chemical welding process in which an exothermic reaction is used to supply the essential heat energy.
• A mixture of fine aluminium powder and iron oxide in the ratio of $1:3$ by weight is used.
• Aluminium has a greater affinity for oxygen than iron and the reduction of aluminium is highly exothermic and gives sufficient heat to melt the metal.

$2\mathrm{Al}\left(\mathrm{s}\right)+{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)\to {\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+2\mathrm{Fe}\left(\mathrm{s}\right)+\mathrm{heat}$

Explanation for the incorrect option:

(B) it is not a strong oxidising agent.

(C) it is corrosion-resistant but does not provide heat to melt the metal.

(D) it is a good conductor of heat but this property is not able to fill the gap in thermite welding.

Hence, Aluminium powder is used in thermite welding because it is a strong reducing agent.