ANS:B - infinite trays and minimum reboiler heat load.

When a multistage tray tower uses a minimum reflux ratio, it implies minimum trays and maximum reboiler heat load. Here's why:

1. Minimum Trays: A minimum reflux ratio means that the minimum amount of liquid is being returned to the top of the tower as reflux. In this scenario, the tower requires fewer trays to achieve the desired separation efficiency. The minimum reflux ratio typically corresponds to the minimum number of trays necessary to achieve the separation specifications.
2. Maximum Reboiler Heat Load: The reboiler is responsible for providing the heat necessary to vaporize the liquid entering the tower from the bottom. When the reflux ratio is minimized, the amount of liquid returning to the reboiler is also minimized. Consequently, the reboiler must provide more heat to vaporize the liquid feed, resulting in a maximum reboiler heat load.