Let's go through each sentence and analyze the appropriate prepositions:
1. When she heard the final verdict, her face took \_\_\_ a grim expression.
The correct preposition here is "on" because "took on" means to adopt or assume, which fits with the context of "a grim expression."
2. I was taken \_\_\_ by the sudden announcement that the meeting was canceled.
The correct preposition here is "aback." "Taken aback" means surprised or shocked, which fits the context of the sentence.
3. The students were taken \_\_\_ with the idea of organizing a charity event for the upcoming holiday season.
The correct preposition here is "up." "Taken up with" means to become interested in or engrossed by something, which fits the context of the students being interested in the charity event.
Step 1: Conclusion.
The correct sequence of prepositions is \( \boxed{(D) CAB} \), where "on," "aback," and "up" correctly complete the sentences.