The correct answer is **challanging** (Option 2). **Comprehensive Derivation:** The correct answer is 'challanging'. Explanation & Key Takeaways: • The correct spelling of the word is 'challenging'. • 'Challenging' is an adjective used to describe something that requires a lot of effort, skill, or determination. • Hence, the correct sentence should be: "The students had to complete a challenging assignment for their science class." Therefore, the correct answer is 'Option 2'. Related Concepts: • 'Students' is correctly spelled. It refers to individuals who are learning, especially those who are studying in a school or college. • 'Science' is also spelled correctly. It refers to a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe. • 'Complete' is spelled correctly as well. It refers to the action of finishing a task or assignment. **Option Elimination:** • **Option 1 (students)**: Incorrect distractor. • **Option 2 (challanging)**: Correct Answer; accurate according to canonical reference. • **Option 3 (science)**: Incorrect distractor. • **Option 4 (complete)**: Incorrect distractor. **Exam Trap:** Be careful of misleading distractors and subtle terminology differences. Verify core properties before selecting your answer. **Memory Booster:** Core mnemonic rule: challanging corresponds to Option (2).