Weight measures
a. how dense an object is.
b. how large an object is.
c. the amount of mass in an object.
d. the effect of gravity on an object.

D) The effect of gravity on an object

Explanation: We're not talking about mass, we're talking about weight. Weight is measured by the force of gravity on a object. For examples, you might be 100 pounds on Earth but 12 pounds on the moon.

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.