In a certain class, the roll number of Dinesh is 10 while that of Uma is 16. What is the roll number of Damyanti?
Which of the below information is additionally required to answer the above question?​​

Dinesh has roll number 10

Uma has roll number 16

We are to find the roll number of Damyanti. 

Option (a): "There are 20 students in the class." (Not sufficient)​

​This tells us the total number of students, but doesn’t say anything about where Damyanti falls in the list.

Option (b): "The roll number of Damyanti is 6 places away from roll number of Dinesh." (Not sufficient)

We cannot find the roll number since it can be six places above or below Dinesh.

Option (c): "The roll number of Damyanti is after Uma." (Not sufficient)​

That just tells us Damyanti’s roll number is greater than 16.

But that’s not enough to know the exact number.

​Option (d): "The number of students with roll numbers between Damyanti and Dinesh is the same as between Damyanti and Uma." (Sufficient)

Roll number of Damyanti
10 + 3 = 16 - 3
13 = 13

So, option d is required to answer the question.