D. The poor living conditions of the working class in industrial cities
Explanation:
Lincoln Steffens is remembered for his work on the corruption and the life of working class in industrial America which he compiled in his book called the Shame of the City.
It delved into the deep seated corruption and systemic exploitation which was rampant in the big industries of the country which were booming with profits but were designed to exploit the common man.
His study on the exploitation on the working class was instrumental in getting more worker rights for them.
