Final answer:
The transition "Also," best clarifies the relationship between the two ideas by indicating the continuation of the theme of courage in different scenarios involving Manjiro and William. The correct answer is option D. Also.
Step-by-step explanation:
If it were added to the beginning of sentence 3, the transition that would best clarify the relationship between the two ideas would be "Also,". This transition indicates that we are adding additional information that is similar to what has been discussed previously, which is that both Manjiro and William show courage, albeit in different circumstances.
In the case of Manjiro, his courage is shown by facing potential danger to be rescued, while William demonstrates courage by presenting his invention despite skepticism from others. By using "Also", we are emphasizing the continuation of the theme of courage in these distinct scenarios.