What is the name of the man who married the Doctor's granddaughter Susan?

The man who married the Doctor’s granddaughter Susan was David Campbell.

Susan meets David during the Dalek invasion of Earth in the 22nd century. He is a human resistance fighter helping defend London, and the two fall in love while working together against the Daleks. Susan initially chooses to remain with her grandfather, but the Doctor recognises that she deserves a life of her own and leaves her behind with David.

The original television story, “The Dalek Invasion of Earth,” shows Susan staying with David but does not depict their wedding. Later Doctor Who novelisations and spin-off fiction establish that they marry, and Susan is consequently often identified as Susan Campbell. The television continuity leaves much of their later life unstated.

The other options are fellow Doctor Who characters: Ian Chesterton was one of Susan’s teachers and an early companion, while Rory Williams and Mickey Smith belonged to later eras of the programme.