Explain one way in which the government has taken action to improve health since 1900

You should look up Progressivism. Progressivists dealt with a lot of reforms in the early 1900s.
Alternatively, try looking up “US health bills since 1900.” If you need a starting point, look up the Pure Food and Drug Act/Meat Inspection Act (both in 1906) and Upton Sinclair’s “The Jungle,” which had a particularly large impact on meat inspection.