Answer:
The correct answer is option D
Explanation:
Half life of the radioactive element is one hour, which means after every one hour its mass reduces by half its previous value
[tex]12: 00 pm\\weight = \frac{40}{2} grams\\= 20 grams\\1:00 pm\\weight = \frac{20}{2} grams\\= 10 grams\\2:00 pm\\weight = \frac{10}{2} grams\\= 5 grams\\[/tex]