Indar is Ani's father. Lakshay is Ruchi's father-in-law and Ani is Tara's sister. Arohi is Indar's paternal grandmother. Tara is Lakshay's granddaughter. If Lakshay has only one child, then how is Ani related to Arohi?

We are given that Tara is Lakshay's granddaughter. 
And we are also given that Lakshay only had one child. 
Along with the information that Lakshay is Ruchi's father-in-law, we can deduce that Lakshay had a son who was married to Ruchi, and they had a daughter named Tara

We are given that Ani is Tara's sister. 
We are also given that Indar is Ani's father, making Indar Ruchi's husband and Lakshay's son. 

Finally, we are given that Arohi is Indar's paternal grandmother. 
Making Arohi Lakshay's mother. 

We can finally get Ani to be Arohi's great-granddaughter. 

Therefore, option C is the correct answer.