In 1981 children in the united states spent an average
of slightly less than two and a half hours a week doing
household chores; by 1997 they had spent nearly six
hours a week.
a. chores; by 1997 they had spent nearly six hours
a week
b. chores; by 1997 that figure had grown to nearly
six hours a week
c. chores, whereas nearly six hours a week were
spent in 1997
d. chores, compared with a figure of nearly six hours
a week in 1997
e. chores, that figure growing to nearly six hours a
week in 1997

D. chores, compared with a figure of nearly six hours  

a week in 1997.

Explanation:

The use of past perfect in the second sentence of the example provided is incorrect since that action developed after the first one. In other words, children spent less than 2 and a half hours a week doing household chores in 1981 and 16 years later (in 1997) this figure increased to 6 hours a week.

a, c, & d are correct:

they wanted lands to settle the extra population.

they wanted new markets for english goods.

they wanted to avoid civil war.