Which group is on the extreme left of the political spectrum, and favors strong government intervention in the economy?
a. conservatives
b. radicals
c. reactionaries
d. moderates

Radicals, communist radicals or socialist radicals. Based on the marxist theory that a proletarian dictatorship is the previous stage of the utopic socialism or communism.

The group which is on the extreme left (but can be also found on the extreme right) of the political spectrum and also favours strong government intervention in the economy would be radicals - b. 

They consider having nationalized companies and a nationalized economy to be an important aspect of social life. 

The extreme right also has radicals who are, however, very different as far as policies go.