What is Tommy and Tuppence's married surname?

Tommy and Tuppence's married surname is Beresford.

Tommy's full name is Thomas Beresford, while Tuppence was born Prudence Cowley. They meet again after the First World War, when both are looking for work and adventure. Their partnership begins in The Secret Adversary, in which they search for Jane Finn and become involved in an international espionage plot.

During that first case, Tommy and Tuppence fall in love and become engaged. Their later adventures therefore follow them not only as investigators but also as a married couple. They establish businesses, work with intelligence contacts and eventually raise a family.

The other options belong to different Christie characters or families, which can make the question deceptively tricky. Tommy is the Beresford by birth, and Tuppence becomes Prudence Beresford through marriage. Their children include twins Deborah and Derek, along with their adopted daughter Betty.