Final answer:
Darwin observed that species vary with latitude, (option B) with biodiversity increasing closer to the equator due to factors like ecosystem age, energy input, habitat complexity, and stability, which may encourage speciation.
Step-by-step explanation:
One of the patterns of biological diversity observed by Darwin is that species vary with latitude. In particular, biodiversity tends to increase as latitude decreases, meaning there is more biodiversity closer to the equator. This is due to several factors, including the greater age of the ecosystems in tropical regions compared to temperate ones, the higher energy input from the sun at the equator, and the stability of tropical ecosystems which might promote speciation. Additionally, the complexity of tropical ecosystems may provide more ecological niches. Therefore, the correct answer to the question is b. Species vary with latitude.