In the period between 1820 and 1860, southerners wanted slavery extended to the western territories so that the south could
a. continue to elect southern presidents.
b. continue to dominate the supreme court.
c. keep enough strength in the senate to protect southern interests.
d. use slave labor to expand southern industries.

The answer is C. keep enough strength in the senate to protect southern interests

Back then, Slavery is almost a way of life of southerners.Back then, the productions amount in northern part of USA is way more than the south. And the Northerners did that without slavery

In 1808 , the congress abolished slave trading with Africa, making the economic difference between the south and north separated  even further

In order to protect their own interest, they want slavery to extend to the north so they have enough strength in the senate to protect its legality