Answer: B) Can't.
Explanation: when speaking about words, a contraction is when two words are combined and some of the letters are replaced by an apostrophe, in order to make the word shorter. From the given options, the one that represents the correct contraction for the word "cannot" is the corresponding to option B: can't, which combines the words "can" and "not" replacing the letters "n" and "o" by an apostrophe.