Correct option is A
The part with an error is
(a) she is the smarter of all.
Explanation: The phrase "the smarter of all" is incorrect. When comparing more than two people or things, the superlative form "smartest" should be used instead of the comparative form "smarter."
Corrected sentence: "Rashi solved the problem in just 2 minutes; no doubt, she is the
smartest of all."