Which of the following was a challenge faced by the earliest overland explorers?
a) an undependable food supply
b) uneasy relationships with native americans
c) the lack of maps to chart their passage
d) all of the above

all of above (100%).

Explanation: there are two specific situations which must be mentioned as quite difficult ...1) winter season was a very harsh and difficult because of food supply, 2) tropical regions (South America and Africa) because of (explorers were Europeans, i.e. accostumed to moderate climate) very high humidity, extremely hot weather and ubiquity of insect.