(D) "effective in . . administered"
Anything that is "effective" results in the desired outcome. "Administered" refers to being given out. The phrase would read "The new vaccine is effective in preventing certain forms of pneumonia and should, therefore, be more widely administered in order to avoid outbreaks of the disease." if these terms were added to the text.
The use of the word "therefore" suggests that, in light of the first portion of the phrase's assertion, the second part of the statement should logically follow. The claim that an effective vaccination ought to be distributed extensively makes sense.
Here's another question with an answer similar to this about passage analysis: https://brainly.com/question/3521530
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