C

As spoken by the Fourth Doctor, C (Morse code: -.-. (TV: The Man Who Never Was)) was the third letter of the alphabet, following B and preceding D. (TV: The Masque of Mandragora)

While under mental attack by the Mandragora Helix, the Fourth Doctor and Sarah Jane Smith spoke the alphabet backwards to concentrate. (TV: The Masque of Mandragora)

When trying to remember "Young Parsons'" name, Professor Chronotis went through the alphabet with the Doctor and Romana II. They started with A, B, and C and eventually got to P, Q, and R before he skipped to X and Y. (TV: Shada, AUDIO: Shada)

As recorded in the Dalek Survival Guide, the Time Paradox Incident hinged on a version of history "C" relative to "A" and "B". (PROSE: Dalek Survival Guide)

The Cybermen of Pete's World had the letter C engraved on their chests to represent the company that created them, Cybus Industries. (TV: Rise of the Cybermen)

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. (AUDIO: The Carrionite Curse, 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)

Cathica Santini Khadeni clarified that her name was "Cathica with a C". (TV: The Long Game)

In 2028, under the influence of Nobody No-One, a cardiogram once sung the alphabet from A to P. (AUDIO: A Death in the Family)

The Tenth Doctor and the entity possessing Sky Silvestry spoke the first fifteen letters of the alphabet simultaneously. (TV: Midnight)