Decomposition breaks down organic material into simpler units and:
rids the world of scavengers
provides oxygen for photosynthesis
provides proteins for plants
rids the world of excess organic material

Option C, provides proteins for plants

Explanation:

Decomposing organism breaks down dead organic material into their base constituents such as nitrogen and carbon and releases them into the environment either in soil or atmospheric air. They decompose the proteins with in the decaying organism and releases energy. Plants absorb these nutrients form soil and use it in performing its cellular metabolic activities. In this way, decomposers not only fix the organic nutrients but also rid the environment from dead organic material.  

Hence, option C is correct