Final answer:
The probability of Adam picking a blue marble is 7 out of 21, which simplifies to 1/3, making '1 third' the correct option.
Step-by-step explanation:
The question asks about the theoretical probability of Adam picking a blue marble from a bag containing 8 red marbles, 7 blue marbles, and 6 green marbles. To find the probability of picking a blue marble, we need to divide the number of blue marbles by the total number of marbles.
The total number of marbles in the bag is 8 (red) + 7 (blue) + 6 (green) = 21 marbles. The number of blue marbles is 7. Thus, the probability of picking a blue marble is 7 (the number of blue marbles) divided by 21 (the total number of marbles), which simplifies to 1/3.
Therefore, the correct option is: 1 third.