Explanation:

- The average of 11 observations is 10, so the total sum of these observations is 110 (since 11 × 10 = 110).

- The average of the first 6 observations is 9, giving a total sum of 54 (because 6 × 9 = 54).

- The average of the last 6 observations is 11, giving a total sum of 66 (since 6 × 11 = 66).

- Observations 1 to 6 and 6 to 11 overlap at the 6th observation.

- If we add the sums of the first and last 6 observations, we get 54 + 66 = 120.

- To find the 6th observation, subtract the total sum (110) from this result (120 - 110).

- The 6th observation is 10.

Correct Answer: Option 4 - 10