D

As spoken by Sarah Jane Smith, D was the fourth letter of the alphabet, following C and preceeding E. (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)

UNIT used Code D to refer to Daleks. (AUDIO: The Dalek Transaction)

A class of sandminer robots were named D for Dum. (TV: The Robots of Death)