Concept:The correct word must match the action of a doctor identifying an illness from its symptoms.
Explanation:"Diagnosed" means to identify the nature of an illness by examining symptoms.
The sentence needs a verb that shows the doctor identified the illness as malaria.
"Discovered" means found something new, not identifying an illness.
"Prescribed" means ordered medicine, which does not fit the structure.
"Announced" means declared publicly, which is not appropriate here.
Therefore, the best word is "diagnosed".
Answer:B. diagnosed