Answer: for justice
In the statement " Many groups worked diligently for justice", the complete prepositional phrase is "for justice". A preposition presents or shows the relationships of the different elements in the sentence therefore, a prepositional phrase commonly is made up of a preposition and an object of the preposition. In the statement above, the presposition is "for" while the object of the preposition is the word "justice". It is very easy to spot a prepositional phrase because they neither have a subject nor a verb but prepositional phrase adds or leads the readers to more information .
