This is a challenging one, for sure. The phrase one hears most often is "cautiously optimistic," which makes total sense. You're being optimistic about the future, but still a bit hesitant and reserved (cautious).
Here, though, you don't have that option. The researcher is reluctant to reach any conclusion based on the limited work done thus far, but adds that they've only just started with the work, inferring, "So who knows what may happen? Maybe this will all prove something after all!"
I do agree with the OA answer. Although, in my opinion, it's not the ideal phrase that would best decribe Premack's attitude, it's the best of the choices given. Premack is being objective in his first statement, saying in effect, "You know, this really doesn't prove anything because it's not an adequately sized sample. This is just one chimp." That's good science, and that's objective. No real scientist would reach a conclusion or consider a hypothesis to be proven based on one test subject. That's scientific objectivity. However, Premack goes on to say, "But then again, we've only just begun working with this chimp, so maybe we will be able to reach a conclusion later." He's tempering his objectivity with hope or thoughts of the unknown future. So the phrase you're looking for needs to bring two seemingly differing ideas together. Only one of the four does that.
While I wouldn't personally call this attitude "cautiously objective," it is the best choice of the four given.
Your choice, "confidently optimistic," doesn't comprise two necessarily opposing concepts. Typically, if someone is optimistic, they are confident that something good will happen, so this phrase is somewhat redundant. That's why the phrase "cautiously optimistic" makes sense -- it's not the normal sort of optimism (which is confident optimism).
I hope this convoluted explanation helps -- it's a bit of a complex topic.