The more specific the topic is, the easier it is to have enough material to talk about it and also, the easier it is to sum up the topic in a single speech. Molecular Biology is a whole science with countless subtopics. And even if Zoology in the Australian outback is more specific, it is still a huge topic. So is "the history of democracy", since it could go over so many countries and the history behind every single one of them. That's why the letter d is the best choice.
