Final answer:
The question asks which of the listed points was not an issue for Darwin's theory in his era. Darwin did face issues with understanding genetic inheritance, and the age of the Earth was underestimated by scientists of his time but not to the extent suggested in the question. However, he could indeed explain how evolution led to an increase in adaptation, so the concept of mutation was not something Darwin knew about, and it did not form part of Darwin's theory during his lifetime.
Step-by-step explanation:
In Darwin's era, his theory had three major issues, but not all choices listed were among them. Darwin did not know how traits were passed on, which meant he did not understand genetics and thus could not explain the exact mechanism of inheritance. This is because the concept of genes and mutations would only come later with the advent of modern genetics. Additionally, scientists of the time thought that the Earth was only around 6,000 years old, not the 12-20 million years mentioned; however, Darwin through his geological observations inferred that the Earth was much older, and this concept was supported by the work of geologist Charles Lyell. Lastly, the ability of evolution to increase adaptation was central to Darwin's theory of natural selection, and he could explain that the traits beneficial for survival would be selected for over time.
Therefore, the correct answer is that Darwin could not explain mutation because the concept was not yet known; all other options were indeed problems faced by Darwin in his time.