Answer: C) difficult.
Step-by-step explanation: Context clues are hints that an author gives to help the reader to understand a difficult or unusual word within a sentence or paragraph. In the given sentence we can identify the meaning of the word "onerous" as "difficult" (corresponding to option C), because the author uses an example as a context clue, in this case, the example is writing a 10-page research project, he describes this task to be "onerous" so we can infer that it is "difficult."