According to your reading, how did the Ottoman rulers promote unity among such a diverse population?

By framing rules and laws and strictly following them

Explanation:

The diversified religious groups were maintained by millets. For each religious group there was one millet

The concept of unity was proclaimed in the article of 1876 of constitution of Ottoman empire. The concept of unity was publicized by the government through newspaper for achieving economic and cultural prosperity.

Some minor measures such as admission to military on the basis of religious sects was prohibited and only deserving candidates were allowed to serve the army