Final answer:
Sydney Carton performs the ultimate personal service for Charles Darnay by sacrificing his life for him, indicating future themes of sacrifice and redemption in 'A Tale of Two Cities'.
Step-by-step explanation:
The personal service that Sydney Carton does for Charles Darnay is that he swaps places with him in prison, leading to Carton's execution in Darnay's stead.
This act of sacrifice hints at future themes of redemption, selflessness, and the power of love and transformation in the narrative.
Carton's act is a pivotal moment in 'A Tale of Two Cities' by Charles Dickens, showing the depth of his character and the drastic changes he undergoes.