To find the number of three-digit numbers of the form xyz where x, y, z are digits, we need to consider the constraints on each digit.

Constraints on x:
- x cannot be zero as the number should be a three-digit number.
- x can take any value from 1 to 9, so there are 9 possibilities for x.

Constraints on y:
- y can take any value from 0 to 9, so there are 10 possibilities for y.

Constraints on z:
- z can take any value from 0 to 9, so there are 10 possibilities for z.

To find the total number of three-digit numbers, we need to multiply the number of possibilities for each digit.

Number of possibilities for x = 9
Number of possibilities for y = 10
Number of possibilities for z = 10

Total number of three-digit numbers = Number of possibilities for x * Number of possibilities for y * Number of possibilities for z
= 9 * 10 * 10
= 900

However, we need to exclude the numbers where x = y = z, as they are not three-digit numbers. For example, if x = y = z = 1, the number would be 111, which is not a three-digit number.

To find the number of three-digit numbers where x = y = z, we need to consider the possibilities for x and y. Since x and y can take any value from 1 to 9, we have 9 possibilities for x and 10 possibilities for y.

Number of possibilities for x = 9
Number of possibilities for y = 10

Total number of three-digit numbers where x = y = z = Number of possibilities for x * Number of possibilities for y
= 9 * 10
= 90

Therefore, the final number of three-digit numbers of the form xyz = Total number of three-digit numbers - Total number of three-digit numbers where x = y = z
= 900 - 90
= 810

The correct answer is option 'A' (120) is incorrect. The correct answer is 810.