Final answer:
Option d, 'Darwin could not explain how evolution led to an increase in complexity' was not one of the major problems during his era. Darwin did not know how traits were passed on and did not know about mutation, but the age of the Earth was generally thought to be much younger than 15-20 million years at the time.
Step-by-step explanation:
In Darwin's era, his theory had three major problems, but one mentioned issue was not among them. First, Darwin did not understand the mechanism behind the inheritance of traits because he did not know about genes. This impacted his ability to explain how characteristics are passed from one generation to the next. Second, during Darwin's time, the age of the Earth was underestimated, with most people believing it was only 6,000 years old. However, the misconception that scientists of the time thought that the Earth was 15-20 million years old was inaccurate, as they actually believed it was much younger. Finally, Darwin was unaware of the concept of mutation, which is a crucial element in the modern understanding of evolution.
The answer to the question is: Darwin could not explain how evolution led to an increase in complexity (option d) was not one of the major problems during Darwin's era. Other scientists, like Lamarck and Lyell, influenced Darwin, but it was the lack of understanding of genetics that presented a significant issue, as this was essential for explaining how evolution operates through changes passed down through generations.