In Goldschmidt aluminothermic process which of the following reducing agents is used :

The Goldschmidt aluminothermic process is a method used to extract certain metals from their oxides. This process involves a thermite reaction, which is highly exothermic and produces enough heat to melt the metal being extracted.

The term "aluminothermic" indicates that aluminum is the key reducing agent. In this process, aluminum powder reduces the metal oxide to its pure metal form, while aluminum itself gets oxidized to aluminum oxide. The general reaction is:

$$\text{Metal oxide} + \text{Aluminum} \rightarrow \text{Metal} + \text{Aluminum oxide} + \text{Heat}$$

For example, chromium(III) oxide is reduced to chromium metal:

$$\text{Cr}_2\text{O}_3 + 2\text{Al} \rightarrow 2\text{Cr} + \text{Al}_2\text{O}_3$$

Now, let's evaluate the options:

Option A: Calcium - Calcium is a reducing agent in some processes (like the reduction of titanium tetrachloride in the Kroll process), but it is not used in the aluminothermic process.

Option B: Coke - Coke (carbon) is used as a reducing agent in blast furnaces for iron extraction, but it is not involved in the aluminothermic process.

Option C: Al-powder - Aluminum powder is indeed the reducing agent in the Goldschmidt aluminothermic process, as the name "aluminothermic" implies.

Option D: Sodium - Sodium is a reducing agent in some reactions (like the reduction of organic compounds or metal halides), but it is not used in this specific process.

Therefore, the reducing agent used in the Goldschmidt aluminothermic process is aluminum powder.

Hence, the correct answer is Option C.