The grammatical problem with the sentence is the following one:
B) redundancy.
The youth group met at 4:00 p.m. to protest against the proposal for an extended school day.
If you protest, you make it clear you oppose to something. Therefore, "against" is redundant. A way to correct the sentence would be by removing "against".
Answer: B) redundancy
Explanation: Redundancy is the grammatical error in the sentence. Using the words 'protest' and 'against' is redundant, which means unnecessarily repetitive.
