Ans: (a)

Let S1 = 1, 2, 3, 4,......., 24,   S2 = 50, 49,........., 25;

New series after interchange S1 = 1, 2, 3, 4........, 25,  S2 = 50, 49,.......24

It is therefore clear that S1 continues to be in ascending order.

As this is a variable based question: the word "ANY" can be used

Let the series of integers $a_1,a_2,.......,a_{50}$ be $1,2,3,4,5,......,50.$

$S_1=1,2,3,4,......24,S_2=25,26,27,.........50$