a. for justice This is the right option.
Many groups worked diligently for justice. The underlined structure is the complete constituent . It is made up of for ( prepostion) + justice ( a complement to the preposition ). This prepositional phrase functions as a beneficiary adjunct and it modifies the whole sentence : Many groups worked diligently...... For justice adds information but it is not essential. This will depend on the context. It is the answer to the question : Who do many groups work diligently for?
