Final answer:
Richard Rodgers and Oscar Hammerstein formed the greatest musical theater writing partnership of the 20th century, creating influential works like Oklahoma!.
Step-by-step explanation:
The musical theater's greatest writing partnership of the 20th century referred to in the question is between the composer Richard Rodgers and the lyricist Oscar Hammerstein. This dynamic duo was responsible for creating some of the most iconic and groundbreaking musicals in American theater history.
Their work included classics such as Oklahoma!, which set new standards for the musical theater genre with its innovative integration of story, music, and dance. Their partnership began with this musical and continued to produce acclaimed productions that resonated with audiences and critics alike.