Writing is the activity or skill of putting words on a page or screen. It can also mean the words themselves that someone has written, as in Her writing has improved, where it refers to the way someone writes. The context tells you which meaning applies: in a sentence like The writing was too small to read, the word means the words that were written on the page, not the activity of writing them.
The definition covers both sides of the word: writing as the activity of putting words down, and writing as the words that result. The activity sense shows up in everyday examples. A text message to a friend is writing, and so is a shopping list like Milk, bread, eggs. Each one is words written down for someone to read.
The most likely answer is A. Please send your writing treats writing as the words that were written, the finished text you can email to someone.
B fails because her writing there means her handwriting, the look of the letters on the page, not the text as a whole.
C fails because the empty notebook has no writing in it yet; nothing has been put down.
D fails because I enjoy writing means the activity of putting words down, not the words themselves.