Which summary of this idea best maintains its formal tone?
A. I always used to get mixed up about Pluto being a planet or not
until I saw the solar system model in a museum that got it right.
B. It took a really long time for museums to show that maybe Pluto
isn't a planet, except for this one museum.
C. Scientists had begun to debate privately whether or not Pluto is a
planet, but this was the first time the concept was presented in a
museum for all to see.
D. Museums might not have Pluto in the right spot yet, even though
they're supposed to, because scientists are pretty sure it's not a
planet.

The summary of the idea which maintains its formal tone is C. Scientists had begun to debate privately whether or not Pluto is a planet, but this was the first time the concept was presented in a museum for all to see.

What is a formal tone?

This is a formal writing common among professional and academic context which focuses on being thorough and direct, yet respectful. It uses full words rather than contractions and emphasizes on facts and grammatical corectness.

The formal tone uses full words here to explain an academic form of writing.

The correct answer is option C.