Final answer:
A) A fungus. The organism that grows inside living trees, digests wood to make it rot, and eventually kills the host tree, entering as a spore through a wound, is a fungus.
Step-by-step explanation:
The organism you're describing is A) A fungus. This organism enters the tree through wounds and establishes itself as a mycelium, a whitish, thready mass. Fungi like this one are key decomposers in our ecosystem and have a unique relationship with their environment.
Fungi are heterotrophs, which means they consume organic matter for their energy and nutritional needs, similar to humans and other animals. However, unlike animals that ingest food, fungi secrete enzymes outside their bodies to break down organic material, such as wood, and then absorb the released nutrients. The damage described is consistent with a wood-decaying fungus, which digests the wood from inside trees, often resulting in what is known as heart rot.
This organism's parasitic nature is evident as it feeds on the living tissue of a host until it eventually kills it. When a fungal spore lands on a tree wound, it has the potential to germinate and form a mycelium, feeding off the tree's resources until it can no longer sustain both itself and the fungus. This type of parasitism is characteristic of many biotrophic fungi, which require living hosts to survive and reproduce, spreading their spores to new hosts to continue their life cycle.