Which factor was most directly related to the decline of the Hapsburg (Austria-Hungary) ?

nationalists sentiments from ethnic groups in the empire

Nationalists sentiments from ethnic groups in the empire, is the right answer.

Explanation:

During the period between the 1500's and 1600's the Hapsburg in various internal as well as external issues such as internal political intrigues and war with the Turks. Besides this, the Bubonic plague prevailed extensively in Vienna in the decade of 1680's. The decade of 1800's saw the decease of the vast empire maintained by the Hapsburg. However, there were many reasons for the decline of the Hapsburg empire but the Nationalists sentiments from ethnic groups in the empire were directly led to the decline of the empire.