Answer: If losing the watch has not altered how we value the watch, then we'll pay the $85 to buy the watch.
We'll pay $85 to buy the watch. since the value of the watch has not been altered and we still value it as the same even after loosing it. Here we are at a surplus of $15 after buying it at a price of $85.
Therefore, We should buy it. The correct option is (b)