Answer:
Option D - It is the product of the initial amount and the decay factor after w weeks.
Explanation:
Given : Ariana starts with 100 milligrams of a radioactive substance. The amount of the substance decreases by 20% each week for a number of weeks, w. The expression
finds the amount of radioactive substance remaining after w weeks.
To find : Which statement about this expression is true?
Solution :
In Ariana's expression,

100 represents the amount of substance he begins with.
and (1-0.2) i.e, 0.8 is the amount of substance left after each time period or the decay factor after w weeks.
The given expression represents the product of initial amount and the decay factor after w weeks.
Therefore, Option D is correct.