Which of these the spread of disease, bands of thieves, poorly established routes or the
hostile ottoman warriors made the land routes dangerous to travel to asia in 1492

I am going to say band of thieves.  Here is why the Ottoman Warriors were not around during 1492 I believe.  And from what I have learned in past years is that bands of thieves would camp along trade routes on land and take cargo being transported.  Such as gold, cotton, clothes.  So in my opinion it would be band of thieves.  But please tell me if im wrong.