Eight friends are sitting around a round table facing the centre with equal distances between two neighbours. Sehaj is sitting opposite to Farhan. Aparna is second to the left of Ruchi. Only Karan is sitting between Tony and Sehaj. Ruchi is opposite to Prerna who is immediate neighbour to Farhan. Harsh is somewhere between Ruchi and Farhan. Who is sitting opposite Karan?

The only directional constraint we have is for Ruchi and Aparan, so that would be a good starting point. 

Placing Ruchi at the bottom and Aparna to the second left to her. 
After that we are also given that Prerna is opposite to Ruichi, making Prerna at the top. 

Net, we are given that Sehaj, Karan and Tony are in a sequence, so they must occupy the right side of the circle. 
As for their positioning, Frahan sits next to Prerna and opposite to that, giving us only one possible configuration, i.e., Frahan is to the right of Prerna, and Tony is to the left of Prerna. 

And the only spot left would be acquired by Karan. 

As we can see, Aparna sits opposite Karan. 

Therefore, Option D is the correct answer.