Answer:
The sentence that is an example of an objective summary is:
B. Malaria was common in marshy areas where mosquitoes bred.
Step-by-step explanation:
An objective sentence, passage, or summary must avoid expressing opinions and feelings concerning the subject. In other words, they must avoid any sort of bias. The author must focus on expressing facts only, his own opinions being left aside. For that reason, letter B is the best option for an objective summary. It is a fact that malaria was common is marshy areas, precisely because it was in those areas that the mosquitoes bred.
On the other hand, options A, C and D express the point of view of the speaker. They are biased, which is shown by the use of the words "interesting", "ridiculous" and "obviously". They can all be eliminated.