Nearly all tribes or states identified outside of Roman territory were
typically in conflict with the
Romans. Which tribes likely posed
the biggest threat, and why?

The answer is d if you are on edgenuity. 

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen