Which tertiary colour can be considered the coolest option that still contains a red influence?

Correct answer: Red-violet

The governing concept is the position of a colour on the traditional colour wheel and the balance of warm and cool influences. Red-violet contains red, but its violet component shifts it toward the cool side. Red-orange and yellow-orange are warmer because they contain orange, while yellow-green has no red component. Therefore, red-violet is the best answer.