Explanation:

- When throwing a dice two times, there are a total of 6 (first throw) x 6 (second throw) = 36 possible outcomes.

- We need the outcomes where the first throw is not less than the second throw.

- Possible combinations include: (1,1), (2,1), (2,2), (3,1), (3,2), (3,3), and so on until (6,6).

- For each number 'n' thrown first, the second can be any number from 1 to 'n'.

- Count the outcomes like: 1 with 1, 2 with 1-2, 3 with 1-3,..., 6 with 1-6.

- The count is: 1 + 2 + 3 + 4 + 5 + 6 = 21.

- Option 3: 21 is the correct choice.