B

As spoken by the Fourth Doctor, B was the second letter of the alphabet, following A and preceeding C. (TV: The Masque of Mandragora)

In November 1963, science teacher Ian Chesterton set his Coal Hill School class a problem with the letters A, B and C representing the three dimensions. However, Susan Foreman pointed out that he had not acknowledged two additional dimensions: D, which Ian reasoned was time, and E, which Susan revealed was space. (TV: An Unearthly Child)

While unable to directly oppose Ruby White, Mr Smith told Clyde Langer "letter two (B), letter five (E), letter twenty three (W)", which Clyde interpreted as "beware". Mr Smith then added "letter eighteen (R)", with Clyde deducing that he was trying to warn him about Ruby. (TV: Goodbye, Sarah Jane Smith)