Despite the fact that New Guinea was one of the earliest settings on earth where wild plants were successfully domesticated, overall food production on the island remained limited for a long time, inhibiting the development of technology and political organization. According to Guns, Germs, and Steel, one of the reasons for this limitation was:
a) There was an overabundance of large-seeded wild grasses suitable for cereal production, so farmers concetrated on cereals and neglected other crops.
b) The crops produced in New Guinea were low in protein content, and the absence of large domesticable mammals only made things worse.
c) The early inhabitants refused to live in the highlands, where there were more resources and domesticable plants available than in the lowlands.
d) As an isolated island population, New Guinea residents were suspicious of outsiders and resisted adopting domesticated crops that were imported from other lands.
Of these options, the best choice is Option B. In fact, Diamond says almost this exact thing in Chapter 8 (and again in Chapter 15). This is on pages 148 to 150 in the paperback edition of the book. On page 149, Diamond tells us that
New Guinea highland farmer populations suffer from severe protein limitation, because the staple crops that provide most of their calories … are low in protein.
He also says, on p. 148, that New Guinea had “no domesticable large mammal species whatsoever.”
This clearly shows us that Option B is the best choice.
Option A is clearly not true. On p. 148, Diamond says that there were no cereal crops domesticated in New Guinea. He says that this is because none of the 56 kinds of grass in the world with the biggest seeds is native to New Guinea. This makes A false.
Option C is clearly not true. There were many people living in the highlands of New Guinea. Diamond talks about these populations, as in the quote above.
Finally, Option D is not true. Diamond notes on p. 149 that New Guineans eagerly used the sweet potato when it was introduced from the Philippines. This cause a population boom in the highlands since sweet potatoes have much more protein than native New Guinean crops do.
Therefore, B is the best choice. Because New Guinea had plants that were low in protein and no animals to help provide more protein, it was not possible for it to sustain the large populations needed for technological and political advancement.