Final answer:
The letter 'C' represents a function when placed on a coordinate system, while the letters 'A,' 'B,' and 'D' do not.
Step-by-step explanation:
Of the capital letters of the alphabet, the only letter that represents a function when placed on a coordinate system is C, horizontally to the right side of the coordinate system. The other letters, A, B, and D, do not represent functions when plotted on a coordinate system.