The function f(p) is used to model the amount of money spent on the project.
The domain is "p" that is the number of people working on this project.
So the question is asking what should be the type of this domain "p"
Let me ask you can the number of people be negative?
Is it possible to say there are -3 people working ?
No, the number of people must be a positive number
Now one more question.
Can the number of people be in fraction?
is it possible to say there are 0.5 people working?
No, the number of people must be whole numbers like 1, 2, 3 etc
So the correct answer is "positive integers"
Because real numbers also include fractions so that is incorrect