Answer:
- A book published in 1970 by a soldier trained to protect elephants.
Out of these options, the best source of information about early efforts to stop elephant poaching would be the book. There are two reasons for this. First, the book was written by a credible author: a soldier trained to protect elephants. Second, the book was published in 1970, which means it discusses the time period we are interested in.
Consequently is the right transition word. This word connects the first sentence (the cause) with the second one (the effect) by showing that the second action took place because of the first action.