I'd say for sure "In conclusion". "Overall" might be one of them as well.
An appropriate summary for the article "A Most Troublesome Element" is the last option - C. Arsenic is a naturally occurring, poisonous element that has threatened the health of humans since the Middle Ages.
It clearly states what the whole article is about.
C. The author uses a simile to compare the feeling of nervousness to melting because the sentence uses the word "like," which is an underlying hint to similes. Without it, it would be a metaphor.
