Which of the following is NOT an example of a capital good?

Elliott buys a pen for his accounting job.
Spencer buys a tractor to help him plow his asparagus fields.
Claire buys a book to help her write poetry.
Daniel invests in bitcoin.

Claire buys a book to help her write poetry. This is not an example of a capital good, as it is a consumable item rather than a durable good used for production.