Step 1: Approach
Recall the standard conditions of the Haber process and what each material does.
Step 2: Details
Reaction: $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$. Catalyst: finely divided iron. Promoters: $\text{Al}_2\text{O}_3$ and $\text{K}_2\text{O}$. Typical conditions: about 200 atm and 700 K.
Step 3: Meaning
The promoters keep the iron surface in a more active, finely divided state, which raises the yield without being the catalyst themselves.
Step 4: Conclusion
So $\text{Al}_2\text{O}_3$ acts as a promoter, option (D).
Final Answer:
Aluminium oxide improves the iron catalyst, so it acts as a promoter, option (D).
\[ \boxed{\text{Promoter}} \]