To express the number "one lakh" in figures, we need to understand the number system used in India. In India, the number system is based on the "Indian numbering system" which is different from the "Western numbering system" used in many other countries.

The Indian numbering system is based on the concept of "lakh" and "crore". In this system, one lakh is equal to 100,000. To write one lakh in figures, we can use the following format: 1,00,000.

Let's break down the figure "1,00,000" to understand it better:

- The first "1" represents the number of lakhs.
- The "00" represents the number of thousands.
- The last "000" represents the number of units.

So, when we write "1,00,000" in figures, we are essentially representing one lakh, which is equal to 100,000.

In the given options, option 'A' (1,00,000) is the correct answer because it correctly represents one lakh in figures according to the Indian numbering system.

Here is a summary of the options and their explanations:

a) 1,00,000: This option correctly represents one lakh in figures.
b) 10,00,00: This option represents ten lakhs, not one lakh.
c) 10,000: This option represents ten thousand, not one lakh.
d) 100,00,000: This option represents one crore, not one lakh.

Therefore, option 'A' (1,00,000) is the correct answer as it correctly represents one lakh in figures according to the Indian numbering system.