I'm studying the Mozart string quartet K. 421 in D minor and I need to discuss the thematic concentration in the piece!

Does Grove give you a definition of "thematic concentration," or do you have a good understanding of that according to Rosen? You may have to delve into his book, The Classical Style (1971), which analyzes the nature and evolution of the high classical style as it was developed by Haydn, Mozart and Beethoven.

As for the quartet, "[it's] one of a set of six dedicated to Haydn, a composer Mozart respected and held in great esteem. Maybe because he knew that Haydn had already proved a receptive audience, Mozart felt uncharacteristically free to open himself up in this music. He completed the first of his Haydn quartets (K.387) by April 1783. The other five were finished by January 14, 1785, and published later that year. All six quartets give us a rare glimpse into Mozart's inner feelings in some of the most private music he was to write."