Answer:
The sentence that has correct capitalization is B. The guide did something amazing: She steered the canoe safely through the flooded river.
Step-by-step explanation:
Sentence B is the correct answer because in American English, according to some style guides, the first word after a colon is capitalized only if it is a proper noun, acronym or a complete sentence, like in this case. Whenever a colon introduces a list of things or simply a phrase, like in sentence A, C and D, one should not capitalize the first word after the colon unless it is a proper noun.