Step 1: Identify the Disease:
"Dum-dum fever" is another name for **Kala-azar** or **Visceral Leishmaniasis**.
Step 2: Identify the Pathogen:
This disease is caused by the protozoan parasite **\textit{Leishmania donovani}**, which is transmitted by the sandfly (\textit{Phlebotomus}).
Step 3: Analyze Other Options:
(A) \textit{Trypanosoma gambiense}: Causes African Sleeping Sickness.
(B) \textit{Ancylostoma duodenale}: Hookworm (Helminth).
(C) \textit{Leishmania tropica}: Causes Cutaneous Leishmaniasis (Oriental sore/Delhi boil).
Step 4: Final Answer:
Option (D) is correct.