Question

what is the meaning of sharp as a tack?
although mr. buchanan is nearly eighty years old, he is still as sharp as a tack.
witty and intelligent
in an uncertain or risky position
able to cut easily
quite old
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Explanation:

The phrase "sharp as a tack" is an idiom. In the context of the sentence "Although Mr. Buchanan is nearly eighty years old, he is still as sharp as a tack", it is used to describe a person's mental ability. "Witty and intelligent" fits this context as it refers to mental sharpness. "In an uncertain or risky position" is about a situation, not mental ability. "Able to cut easily" is a literal meaning of "sharp" but not the idiomatic sense here. "Quite old" is the opposite of what the idiom implies in this context (the sentence is highlighting that despite being old, he has mental sharpness).

Answer:

witty and intelligent