Protons are located in the nucleus of the atom. a proton has
a) no charge
b) a negative charge
c) a positive and a negative charge
d) a positive charge

d) A positive charge  

Explanation:

This is the definition of a proton: a particle in the nucleus that has a positive charge .

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.

the answer to this question is 3.