The phrase "pen it down" means **to write** because in this context, "pen" is used as a verb meaning "to write or compose," and "down" suggests putting thoughts onto paper. The passage ends with the instruction: *"Think of a plot—make points on how the story will progress and pen it down,"* which clearly encourages the reader to write the plot points on paper. The other options—"to writ" (an archaic or incorrect form), "to throw the pen," or "to throw it down"—do not fit the logical or grammatical meaning of the sentence.