Question mark

The question mark, ?, was a punctuation mark used in many documents on Earth to denote an interrogative.

As Fashion for The Doctor
During some of his incarnations, the Doctor adopted the question mark as a personal motif. The Fifth and Sixth Doctors displayed question marks on the collars of their shirts. (DW: Castrovalva, The Twin Dilemma)

The Seventh Doctor wore a jumper covered with the symbol (DW: Time and the Rani), carried an umbrella with a large red question mark-shaped handle (DW: Delta and the Bannermen), a business card with a stylized question mark and when asked to sign his name, did so with a question mark. (DW: Remembrance of the Daleks