Final answer:
The correct definition for the function is option d) 2x, which indicates that the output of the function is two times the value of x.
Step-by-step explanation:
The definition for the function 2x is best represented by option d) 2x. The options a) and b) are conditions that could be applied to when a function is used rather than its definition, while option c) represents a different function, specifically x + 5. Therefore, the correct definition for the function in question is simply 2x, which implies that for any given value of x, the function's output will be the product of 2 and x.