The answer that correctly uses the colon is the last sentence. Normally a colon is meant to be used when it is showing a list of options, but the first and second sentence only have one sentence after that. A colon also can show reasons why a person does something, but for the third option, there's only one reason. In the last statement, the colon is used to break up the statement. It also shows that Harry has something in account for what he wants in his life. So, your answer option should be "D"
The sentence which contains a compare and contrast text structure is C, as in this sentence the comparison and contrast is between plant cells and animal cells.
