Final answer:
A comma is needed in the sentence, 'In 1879, Louis Sullivan and Dankmar Adler formed the innovative architectural firm, Adler & Sullivan.'
Step-by-step explanation:
In 1879, Louis Sullivan and Dankmar Adler formed the innovative architectural firm, Adler & Sullivan.
In this sentence, a comma is needed after '1879' to separate the introductory phrase from the main clause. The main clause is 'Louis Sullivan and Dankmar Adler formed the innovative architectural firm, Adler & Sullivan.'