Final answer:
In Act V of 'Hamlet,' Rosencrantz and Guildenstern are most likely dead due to Hamlet altering the letters meant to order his execution to command theirs instead.
Step-by-step explanation:
In Act V of Hamlet, the titular character explains that Rosencrantz and Guildenstern are most likely dead. This is revealed in the play when Hamlet alters the letters they carry to England which originally contained orders for Hamlet's execution. Instead, Hamlet ensures the orders call for the deaths of Rosencrantz and Guildenstern, thus sealing their fate without their knowledge.
The changing of the letters is a critical moment demonstrating Hamlet's cunning and also serves to move the plot toward its tragic conclusion.