What occurs when a spell instructs players to exchange life totals?

When a spell instructs players to exchange life totals, the result is that each player adjusts their life total to match the other player's life total. This means that the player with the higher total will decrease their life to match the lower total, and the player with the lower total will increase their life to reach the higher total.

This mechanism is a straightforward application of how life totals interact with spells and abilities that involve exchanges. The rule concerning life total exchanges ensures that the transition happens seamlessly, and both players will end up with the same life total, regardless of their starting points.

The other options do not accurately describe the outcome of an exchange of life totals. For instance, gaining the same amount of life doesn't capture the essence of an exchange, and discarding cards based on life totals does not occur in this context. The assertion that life totals cannot be exchanged contradicts the fundamental capability of certain spells designed to perform this specific action. Thus, the accurate understanding here is that life totals are swapped directly between the players, leading to the situation captured in the correct answer.