Final answer:
The Incas did not have a unique written language; their record-keeping system used quipus, which were colored strings with knots to convey information.
Step-by-step explanation:
The correct answer to which of the following was NOT unique about the Incas is c) Their written language was a system of knots called quipu.
The Incas, like many other civilizations, developed a sophisticated record-keeping system featuring quipus, which involved a set of colored strings with knots that were used to convey information. However, this was not a written language.
Each of the other options: a) their massive system of roads, b) worship of a pantheon headed by a sun god, and d) being the largest empire in the Americas, were indeed distinctive traits of the Inca Empire.