The correct option is C ancient
The word 'modern' means 'relating to the present times'.
Option A: 'Happy' is a feeling of emotions or contentment, whereas 'modern' refers to the present time period.
Option B: 'Noble' refers to a person who is has high moral values.
Option C: A strife is a quarrel or a fight.
These words do not give the opposite meaning of 'modern'. Hence options A, B and C are incorrect.
Option D: The word 'ancient', as mentioned in the line "Ring out a slowly dying cause, and ancient forms of party strife" means 'belonging to a very distant past'.
Ancient means the opposite of 'modern' as the former refers to the old whereas the latter refers to the new. Hence D is the correct option.