Which historical factor explains the leading religion in albania, kosovo, and bosnia and herzegovina (the countries shaded dark orange)?

The correct answer is - A. The Ottoman empire once controlled these countries.

The Ottoman empire controlled these territories for around five centuries. During that period, the Ottomans managed to force and convince some of the population to practice their religion. The Ottomans were practicing Islam, Sunny Islam to be more specific, so some people in what are now Albania, Kosovo, and Bosnia and Herzegovina became Sunny Muslims too, mainly in order to have some benefits like avoiding or paying less tax.

Over time, because of the higher birthrate, the Muslim population became the dominant one in these countries, so the once Catholic Albania, Orthodox Christian Kosovo, and partially Catholic partially Orthodox Christian Bosnia and Herzegovina became predominantly Muslim countries.