Why does vegetation vary from tropical temperate forest to tundra in mountainous areas

The vegetation in tropical temperate forests varies from that of tundra in the mountainous regions mainly because the climatic conditions there greatly vary from each other.

Explanation:

The tundra climate that is characterized by the negligible presence or total absence of heat favors the growth of only a few species of grasses, moss and lichens. Whereas, the climate where tropical temperate forests stand allows it to host over hundreds of plant species. Example: Rubber trees, Mahagony trees, Cacao trees, etc.

I'm pretty sure it's c. extremely gradual but not 100% sure though.

Answer any resource, such as wood or solar energy, that can or will be replenished naturally in the course of time;