The decimal d is formed by writing in succession all the positive inte

Look at the question one sentence at a time:

"The decimal d is formed by writing in succession all the positive integers in increasing order after the decimal point; "

This is how decimal d is formed - write all positive integers (i.e. 1, 2, 3, etc) in increasing order

"that is d = 0.123456789101112"

Here they have shown what they mean
d = 0.123456789101112... and so on goes the decimal

"What is the 100th digit of d to the right of decimal point ?"

If you continue putting down positive integers in increasing order, you will get a decimal with infinite digits to the right of decimal. What will be the 100th digit?
d = 0.1234567891011121314...
first digit after decimal is 1
second digit is 2
third digit is 3
10th digit is 1
What will be 100th digit?
First 9 digit will be 1 to 9.
Next 90 digits will come from the next 45 two digit numbers. The next 45 two digit numbers are 10 - 54.
We have accounted for 99 digits. The 100th digit will be the 5 of 55.