Choose whether the reaction is exothermic or endothermic. 4Al(s) + 3020w 2Al2O3(s) --Select--
V
2H2(g) + O2(g) → 2H2O(9) --Select--
N2(g) + O2(g) - 2NO(g) -Select--

the answer is definitely c.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

it's all four options.