In the battle between UK telecom giant Vodafone and its Indian partner Essar, both Vodafone and the Essar Group have appointed Goldman Sachs Group and Standard Chartered respectively for the evaluation purpose. But what is the bone of contention? The crux of the battle is the put option that Essar is entitled to exercise before May 7, 2011, whereby it can sell its entire 33 per cent stake to Vodafone for $5 billion or else can offload partial stake at a fair market value arrived by bankers.