Which of the following best describes the effect of the Battle of Manzikert in 1071 CE on the Byzantine Empire?

The theme system was already in place before the Battle of Manzikert, having been established in the 7th century as a means of organizing military and civilian administration. Although the defeat prompted military reforms, the introduction of the theme system is not directly linked to the battle itself.

While the weakening of the Byzantine Empire post-Manzikert set the stage for later Crusader incursions, the Battle of Manzikert did not directly enable the Crusaders to seize Constantinople. The Fourth Crusade in 1204 ultimately resulted in this event, which occurred much later.

The Byzantine Empire's control over Bulgaria was not directly affected by the Battle of Manzikert. The conflict primarily affected Anatolia and did not immediately alter the Byzantine territorial situation in the Balkans.

The Theodosian Walls were constructed well before the battle, specifically in the 5th century, as a fortification against various threats. The battle itself did not result in the construction of new fortifications but rather showcased the need for existing defenses due to the rising threat from the Turks.