Arun is the _________ child of Mr. Mansingh.

Let A, M, R, N, S, T, K and J represent Arun, Mahi, Rohit, Nilesh, Saurav, Tammana, Kuntala and Janaki.
Let us formulate the given information is a table form:-

We are given that, Saurav is the eldest child and Janki is the youngest.
Thus, S is 1 and J is 8.
Rohit who was born just before Janki, plays hockey. Thus, Rohit is 7 and plays hockey.
Three sons of Mr. Mansingh were born first followed by two daughters.
Thus, 1,2,3 are sons and 4,5 are daughters.
Kuntala, the eldest daughter, plays chess.
Thus, Kunatala is 4 and plays chess.
The updated table is:-

Tamanna and Rohit study at St. Stefan school.
Mansorover school offers cricket only, while Trinity school offers chess.
Thus, Kuntala studies in Trinity.
Mahi and Nilesh are cricketers while Arun plays football
Thus, Mahi and Nilesh studies in Mansarovar.
we are given The children who are at Mansorover school have been born in succession and thus, Mahi and Nilesh are 2/3.
Thus, Tammana is 5 and Arun is 6.
Thus, the table is:-