Answer
B D H

The passage suggests that the human brain was not evolved for modern life in societies with a huge number of people. Now let's look at each of the blanks.

It is difficult to answer the first blank in the beginning. So let's answer the second and third

Blank (ii)It compares and contrasts the population densities in modern cities to the small tribes that early humans lived in. So in contrast to the "small" populations that humans were used to, the population density is significantly higher now. What words signify a copious amount? Nebulous (which means newly formed) and negligible (indicating small amount) clearly do not fit. "Daunting" fits best

Blank (iii) In the preceding sentence, the author states that when living in big societies, many more events become possible. So, if the size of the society is large enough, we can reason that events that would have been considered rare would become "much more than common" ( as indicated by 'not merely common, but..')
What word best fits? "Expected" means that it's almost considered to be normal. Hence it fits best here

Going back to blank (i)
The blank refers to a property of the human brain that evolved when living in small communities. "Cantankerousness" refers to being argumentative. There is no indication given that people were more/less argumentative in the passage. "Evanescence" means fleeting/ temporary. There is no indication that human thought/memory was more/less long-term. Which leaves us with "stubbornness" We are told that the human brain "evolved to focus on the peccadilloes (small mistakes)" Stubbornness can describe this. Hence stubbornness is correct