"cautiously objective" vs "confidently optimistic"

[ilovepsycho]

The chimpanzee may be the animal closest in intelligence to man.
How close? Scientists have often tried to discover in chimps the skill
that makes man different from all other living things: language.
Until now, they have failed; the chimp simply does not have the
ability to speak that every human child is born with. But psychologist
David Premack of the University of California has succeeded in
showing that the chimpanzee can talk with man in ways other than
by the tongue.

Premack's proof is Sarah, a female chimp. Sarah can not only
understand the meaning of 120 sign words, but can use them to build
sentences of her own. After two years' work of only one chimpanzee,
David Premack naturally does not claim anything more than that
Sarah has shown intelligent use of language. "This does not mean
that she can produce all the functions of language, or that she can do
everything a human can," he writes, "But then," he adds hopefully,
"we have only been working with her a relatively short time."


Q. What is Premack's attitude in his quoted statements?
a. arrogantly subjective
b. cautiously objective
c. confidently optimistic
d. reluctantly pessimistic

I've chosen 'c. confidently optimistic,' but OA is 'b. cautiously objective.' I'm a bit confused between the two. Please explain to me.