C

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

As early as his sixth incarnation, the Doctor kept a "C chest" aboard his TARDIS, named for containing a cache of items marked by the letter C. (TV: The Unicorn and the Wasp)

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)