Question

one of the first things vladimir i did after he became the ruler of kiev was to
force the population to submit to his authoritarian rule.
make orthodox christianity the state religion.
isolate kiev so that trade virtually stopped.
create a strong military for kievs defense.

Explanation:

After becoming the ruler of Kiev, Vladimir I made Orthodox Christianity the state religion. This was a significant move that had a profound impact on the cultural and religious identity of Kievan Rus'. It helped in unifying the people under a common religious framework and also had political implications as it aligned Kiev with the Byzantine Empire (which was Orthodox Christian). There is no historical evidence to suggest that his first act was forcing an authoritarian rule (he was more focused on religious and cultural integration), isolating Kiev (trade was important for Kiev's prosperity and continued under his rule), or creating a strong military for defense as the first - thing (though military was important, the religious conversion was a more immediate and notable early act).

Answer:

make Orthodox Christianity the state religion.