Explanation:

Let’s break it down:

- The sum of all natural numbers from 1 to 12 is 1+2+...+12.

- Well, the sum formula for the first n natural numbers is n(n+1)/2. So:

12 × 13 / 2 = 78

- He made a mistake and added one number twice, so his total is 80 (which is 2 more than 78).

- So, the number he added twice must be 2, since 78 + 2 = 80.

Now, here are the options:

- 1, 3 — Adding either 1 or 3 doesn’t get us to 80.

- 2 — This fits: if he added 2 twice, sum becomes 80.

- 7 — 78 + 7 = 85, too high.

- 8 — 78 + 8 = 86, again, too high.

So the answer is Option:2, 2.