Which is the correct use of the word “emerged”?
west african kingdoms emerged to form one union.
trade emerged as a main source of income.
gold emerged from one merchant to another.
which merchant will best emerge the wealth?

The correct answer is: Trade emerged as a main source of income.

The verb "to emerge" means, in this scenario, becoming apparent, important. So it would mean that now, trade has become the main source of income.

Also, all the other sentences are not making the most appropriate use of the verb, being best substituted by other verbs.

the best description of postwar writers might be progressive.

following the war, writers showed a tendency to work on artistic or social matters rather than political themes. while postwar literature may not have been the greatest period of english literature, the themes provided excellent critics a high number of novels. such examples include anthony burgess, anthony powell and frank kermode, all of which can be placed under the progressive view.

food chain i would say only because there is competition on getting food and survival