Final answer:
Option c: if not(turtle.isdown()): turtle.pendown() is the correct Python code to determine if the turtle's pen is up and to change it to down if needed, in the Turtle graphics library.
Step-by-step explanation:
The correct answer to the question Which of the following will determine if the turtle's pen is up and will change it to down if that is the case? is option c: if not(turtle.isdown()): turtle.pendown(). This Python code snippet is used in the context of the Turtle graphics library, where turtle.isdown() is a function that returns True if the pen is already down (and thus the turtle is drawing when it moves) or False if the pen is up (and the turtle is moving without drawing). The if not statement checks if the pen is not down and if so, the turtle.pendown() command is issued to put the pen down and start drawing.