Tarun is the son of Charan. Charan has only two children. Tarun is the brother of Ravali. Ravali is the daughter of Pranavi. Aditi is the granddaughter of Pranavi. Sundar is the father of Aditi. How is Sundar related to Tarun?

Let's trace the relationships step-by-step to build the family tree:
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Tarun is the son of Charan (Charan→Tarun(+)).
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Tarun is the brother of Ravali (Tarun(+)↔Ravali).

Ravali is the daughter of Pranavi. Since Tarun and Ravali are siblings, Charan and Pranavi are married to each other (Charan⇔Pranavi).
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Charan has only two children (Tarun and Ravali).

Aditi is the granddaughter of Pranavi.
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Sundar is the father of Aditi. Since Charan and Pranavi only have two children (Tarun and Ravali), Aditi must be the child of either Tarun or Ravali.

Since Sundar is the father of Aditi, if Aditi were Tarun's child, Tarun would be the father (but Sundar is the father). Thus, Aditi must be the daughter of Ravali, and Sundar must be married to Ravali (Sundar(+)⇔Ravali(−)).

Therefore, Sundar is the husband of Tarun's sister (Ravali), making Sundar the Brother-in-law of Tarun.