Answer: A [regretful]
When trying to identify the tone of a passage, you must look for keywords that highlight the possible emotions the author is trying to illustrate.
What are keywords?
Keywords are words that act as a type of code or hint to help indicate the summary of a statement and its contents. They don't only have to be limited to words, but phrases as well.
Lets start at the beginning of the passage:
(1) Throughout history, people have suffered from ailments that could have been easily avoided if they had only been understood.
In this sentence, a phrase that sticks out the most is "could have been easily avoided if they had only been understood". This phrase, especially the use of "could have" indicates that if certain solutions were discovered, many lives would have been saved. However, the opportunity was missed out on, which has led to some sort of underlying guilt or shame.
Let's take another sentence:
(4) Sadly, the hatters did not know that the mercury they used in creating felt hats was poisoning them, leading to their strange symptoms.
The use of sadly, at first glance, could have been read in a tone of sadness or sorrow/unhappiness. However, when use as an introductory adverb, it takes the definition of "unfortunately" instead, which can also be translated as " regrettably"
Always look for key words when trying to identify the tone of a passage!