The largest one-digit number is 9. Let's understand why this is the correct answer.

Understanding one-digit numbers
One-digit numbers are numbers that range from 0 to 9. These numbers are called one-digit numbers because they are represented by a single digit.

Comparing the options
Let's analyze the options provided in the question:
a) 1
b) 9
c) 0
d) 10

Option a) 1
The number 1 is a one-digit number, but it is the smallest one-digit number, not the largest. Therefore, option a) is incorrect.

Option b) 9
The number 9 is the largest digit within the range of one-digit numbers. It is greater than all the other digits: 0, 1, 2, 3, 4, 5, 6, 7, and 8. Therefore, option b) is the correct answer.

Option c) 0
The number 0 is also a one-digit number, but it is the smallest one-digit number, not the largest. Therefore, option c) is incorrect.

Option d) 10
The number 10 is not a one-digit number. It is a two-digit number because it consists of a digit in the tens place (1) and a digit in the ones place (0). Therefore, option d) is incorrect.

Conclusion
Based on the analysis above, we can conclude that the largest one-digit number is 9 (option b). It is the highest digit within the range of one-digit numbers.