Step 1: Understand the context
- The teacher feels positively about the students' progress.
- The blank should be filled with an adjective expressing positive reaction.
Step 2: Analyze options
- (A) "impressed'' means feeling admiration, fitting here perfectly.
- (B) "depressing'' means causing sadness, opposite meaning.
- (C) "expressive'' means showing feelings clearly, unrelated here.
- (D) "aggressive'' means hostile or forceful, unrelated.
Step 3: Conclusion
Option (A) is the best fit as it matches the positive sentiment.