Explanation:

- Initially, the average of 10 observations is 21. Therefore, the total sum of these observations is \(10 \times 21 = 210\).

- A new observation is added, making it 11 observations in total.

- The new average is 1 less than the previous average, so the new average is \(21 - 1 = 20\).

- The total sum of the 11 numbers is \(11 \times 20 = 220\).

- To find the 11th observation, subtract the original total from the new total: \(220 - 210 = 10\).

Since the 11th observation is 10, the correct option is:

- Option 2: 10

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